Top 20 Witnesses have the biggest responsibility due to the small time-delay between blocks.
So that is a reason to ensure that the system that puts the top 20 in their position is agile enough to ensure that these important positions can be filled by the most appropriate accounts.
And as a developer, there is the saying: Never change a running system
As a system analyst and architect, there is the saying: Systems must always adapt, since change is a constant ;)
Having the same Witnesses in the top 20 (50) slots is unfair for the rest of the Witnesses, but the longer a Witness is in those spots - the more he has proven himself that he keeps his/her witness-node running smoothly.
Not necessarily, that is the whole point here, since the longer someone stays in that top spot, the more resources they have to use to solve any problems and possibly even just pay someone else to solve them. In some senses it just proves they have been in that top spot. Yes, they must have had reliable hardware online for a decent period of time and I am not saying that they are unreliable - I am just saying that being in the top 20 is about much more than just reliability and being outside of the top 20 does not mean you are less reliable. Many top 20 witnesses have a large number of missed blocks, for example.
And objectively - there is no reason to change the top 20 Witnesses as long as those are not f*cking up repetitively.
Witnesses choose hard forks and the top 20, afaik, have the lion's share of the say - so saying that there is no reason to change them unless they (subjectively) fuck up, is like saying that there is no reason to change who runs a government unless they keep failing to turn up to work.. It is quite possible for their decisions or indecisions to go un-noticed for quite some time. Again, this is not a comment on any specific witness, It is an observation and understanding that relates to the system as a whole.
Now, regarding your argument that the Top 20 rewards are unfair:
The own Steempower plays most of the time only a tiny role in the witness ranking.
They are being paid a lot of money and i must be going somewhere. I'm sure they have projects to fund and bills to pay, but there is nothing stopping them from putting their money into second accounts and voting for themselves without you knowing and besides that, their hardware reliability comes with a high price that is beyond the reach of most witnesses (and this will only continue to become more noticeable as RAM requirements grow).
Additionally, the biggest influencers on Steem aren't even running a Witness-node themselves (excluding @blocktrades) - at least afaik.
Do you know who actually controls the biggest influencing accounts?