The problem with approaching the apogee of corporate evolution in EVE online seems to be that the faster you reach the zenith, the less people actually play actively and the more people log in once a week, or only when a Jabber ping goes off.
This is why BUGRY never has and never will go up to C5 space.
There are so many corporations in C5 space which reach 100-200 members where (even counting the fact 1/3rd are alts, 1/3rd are inactive) no one signs in, or only signs in in order to talk shit on comms and POS spin until a scout gets something going and then they wake up, ship up and follow primaries. It's either that or they log on to run escalations, make their billion for the week, and sign off.
Hard Knocks and friends have the right idea, to some degree, with their 'rental' program. The main corporation is made up of peak-level players who are in it for PVP. They cull the inactive, the dullards, the lazy layabout shitheels regularly. People bear in their bear dens, solo escalating or using a smaller alt corp with a few other HK or other members, to make their money outside the main HK wormhole. This efficiently separates the carebears from the PVPers. The main wormhole is never locked down for sites, people don't have to have a bearing capital or dread, or even refits for them.
Likewise, because people are split off into their own individual bear holes, they get to access a better average weekly site spawn rate per toon, maximising income. They can log on for 3-6 hours a week with their bear toon, roll themselves into their hole and smash their home sites in relative safety and be paid up for the week, and some.
Why do i mention this model? Well, aside from the fact it is a sensible response to income management, and also excluding the fact that if a bear hole gets sieged then HK can rage roll to "defend" the bears AKA 'get content' defending the alts, it focuses the main corporation on PVP and ensures activity levels are high.
The problem with the way the majority of C5 operations are run is that they eat where they shit, PVP and PVE wise.
Now, not everyone can technically or financially afford to find an empty C5 and jack a tower up, bring in a solo dread or a small group of alts to escalate....but this is true of basically any corporation anyway. C4-C5-C6 PVE is a group activity, requiring multiple toons online (usually) and requiring multiple people online, to make the billions. If corporate numbers and activity dips particularly low you can often find yourself unable to field the minimum escalation fleet, or not the right toons and not the right ships, which sees the active members stranded for things to do.
Likewise, mixing PVE and PVP means that your PVPers are frustrated because your have home fleets escalating and quavering at the idea of an inbound connection opening up and putting their 25B ISk escalation quad at risk.
Finally, C5 life to the uninitiated is a golden brick road lined with hundred dollar bill lawns where diamond encrusted unicorns graze and shit rubies. People want to be space rich and will readily join up to C5 corporations and sign in only to do sites, or sign in and constantly ask to do sites, why can't we do sites, come on come on. Siiiites.
When the sites don't happen, or there's not enough people online, when chains are full of probers and transients and inbound holes, when the logistics run needs to be done and time is tight, and you can't do sites and make billions...they aren't interested.
So, that's why we haven't and won't go up to C5 space. Sure, C4 ISK isn't spectacular, and you often suffer a fair amount from the lack of numbers to run C4 PVE content anyway (unless you Marauder up, but that's like running escalations as far as hole control goes) but because it isn't spectacular ISK you don't get the carebear element as much.