EVE Fiction

The narative that null sec entities are becoming more powerful than empires is just told so they can feel important.
Don't think such thing will and should happen as a part of the fiction.

The Capsuleers getting more powerful then the Empires is a story that's been developing for a long time now, but capsuleers are still far from challenging the empires as a power themselves.

Military, the large capsuleer groups can match the empires in might and even exceed them in some ways. However, the capsuelers, even those in null are currently too dependant on the empires for resources, manpower & production for their fleets.

On top of it, capsuleers can field massive fleets, because they don't have to patrol the systems & borders, unlike the empires do. Capsuleers can get away with it due to the sparse population. If empires had no need to patrol systems, they could field formidable fleets as well. The Coronation of Empress Jamyl showed a small part of the Imperial fleet on it's video.
Also, remember, there's Concord & empire controlled cyno jammers in every highsec system. While they can get shut down by attacking Concord (see Elder Fleet attack, Caldari prime battle & Colelie incident) they can also get online fast, trapping hostile ships, hence attacks on highsec rarely involve more then a handful of ships. Never deploy more then you can afford to loose.
If nullsec entites would face a similar issue in attacks, I doubt they would field hundreds of capitals in a single attack, only to see them getting trapped when the cyno-jammers get back online after a few hours.

Also, Concord has a firm control over us, wether you like it or not. They can shut us down at a whim, should such extreme cause as you mention ever happen (united front of capsuleers against empire). However, shutting us down is not something that'll happen anytime soon, simply because capsuleers would find a way around it. It's already known that there's capsuleers or capsuleer-like entities existing beyond Concord control.
Example is Sansha's Nation, the various pirate capsuleers (the NPC's) and now the Burner NPc's (who hint at some experimental tech involving capsules, but better )

Last but certainly not least, why capsuleers won't ever attack the empires: self-regulation. Attacking empire space (and vice versa, empires attacking Nullsec) would consume so much resources, manpower & time that competitors would readily attack & take over such weakened aggressor.

So while capsuleer power is growing, they, for now, lack the foundation to challenge the Empires. They have the military power, but lack the requirements to sustain a prolonged attack on the empires (as that means existing while being cut off from all things highsec)
The Great Wars (BoB vs Goons) do show capsuleers can sustain a prolonged war, as long as they can draw upon the resources of highsec. To do the same without empire resources to draw upon is unlikely

PS Bonus info: Jovians & Caldari State do recognize the large sov-holding alliances as independant souvereign states (this came to light during the Jovian ambassador trip years ago) However, the others do not!

Single greatest hurdle with null taking over empire would be, I imagine, to get nullsec powerblocks to work together and not stab eachother in the back when the opportunity arises.

A combination of the logistical cost of doing so, along with the heavy presence of NPC pirates factions. The Amarr Empire had explicit plans to colonize Providence and make it a new border region of the Empire, but the defeat at Vak-Iatoth and subsequent Minmatar rebellion demanded vast quantities of the Empire's resources be refocused on protecting other borders instead, putting those plans on indefinite hold. The Caldari currently have quietly managed facilities in Pure Blind, but no significant colonization plans. In general, inter-empire conflict has consumed resources that might be otherwise spent on expansion.

For an NPC nation to invade and colonize a nullsec region involves massive logistical difficulties that - for reasons of game design and unplanned power creep - we as players never encounter. We have no planetary civilization that demands constant protection and patrol; for game design purposes, NPC pirates ignore our PI colonies even if we never put a combat ship within a jump of them. Asteroid mining is easy beans for us because unfortunate stat power creep has rendered NPC pirates a joke to even the most basically fitted ships. Like PI colonies, our moon goo towers - and, indeed, every other kind of POS including supercap production and jump bridge ones - are utterly ignored by NPCs no matter the value. Game design has removed the need for us to freighter in vast quantities of food, personnel, spare parts, and other materials necessary to support new colonies. It has also granted us the ability to easily locate and target critical enemy capsuleer facilities without even needing scan probes - stations are identified as soon as we enter system, and finding a production POS is as easy as warping to a moon and taking a look inside the bubble.

None of this would be realistically accurate for a colonization effort. Moving in would require rooting out dozens if not hundreds of well-prepared pirate bases none of which are instantly identified at the beginning of a conflict. Simultaneously, constant patrols and defense for support facilities, colonized planets, massive convoys, and belt mining efforts would need to be provided; all of this would demand resources be diverted from other projects which are likely to have political backers of their own, and also the removal of fleets for protection from other regions.


tl;dr - Game design and power creep have left us in the unique position of needing far fewer resources to control a nullsec region, in large part because for us "controlling" only means meeting certain CONCORD-designated requirements rather than actively colonizing and protecting resources in a region.