For example, an 8 million steem account splitting stake into 4 accounts now only gives them 2 million per account to vote with, a number that is much easier to match by other voters on other candidates

That's simply not true because other accounts would also have fewer votes and end up spreading their votes more thinly. The overall vote weight needed to reach the top 20 would decline by about a factor of 6 (assuming most of the stake deploys 30 votes and would deploy 5 given your proposal, which is close to correct, if not 100% accurate), which means high stakeholders with a split of 1/4 would, in immediate terms, have an easier time getting their preferred candidates in.

Of course, 'immediate terms' is never the correct analysis anyway (and your earlier point about other changes not having real papers and proofs is true in most cases–although the curve change does have some–but they all still have and have had extensive, if somewhat informal, analysis and discussion that goes beyond the first level analysis of assuming that nothing else changes and considers at least reasonable suppositions about how behavior might change), but you see the basic idea.

but when we have 2 of the largest stake holders basically deciding all of the top 20 witnesses

And I flat out disagree this is the case. What we actually have is:

a) witnesses voted by at least one of those stakeholders who are not in the top 20
b) witnesses voted by one of the top two who who still wouldn't be in the top 20 (in a static model) even if the other top-two also voted for them
c) witnesses not voted by at least one the top two, who are in the top 20
d) not a single top 20 slot accessible by a combination of the two largest stakeholders alone
e) at least one top 20 witness not voted by either of the top two stakeholders
f) at least one other top 20 witness who would still be there (in a static model) without any top-two votes.
g) etc.

The voting results are determined by a combination of votes from the largest stakeholders as well as the others. According to (d), which has always been the case AFAIK, without a lot of votes from other stakeholders, the top two stakeholders can not even come close to electing even a single witness at all, much less all witnesses. Yes, the largest have a lot of influence, but they are not 'dictating' anything. That's a combination of conspiracy theory, misunderstanding how voting works, and/or trolling.

You are confusing 'most top witnesses have a vote from a top stakeholder' with 'top witnesses are decided by top stakeholders'. These may seem close enough for rhetorical purposes but in terms of how voting actually works they are very different. It would actually be odd and probably dysfunctional if most top witnesses didn't have the support of most large stakeholders, including the largest.