Nobody can quickly add a few changes as it suites them and I doubt if the same can be said about Ethereum.

I think more people would regard that a bonus than they're willing to admit. It's cool to say you love decentralisation until it starts to impact on you, then you'll fall into the arms of the nearest smooth talker who'll slide your panties off and centralise you before you can hit your rape alarm.
Decentralisation is an inconvenience, commitment and long game that eventually pays off in spades when you sit back and think about it. Most people aren't strategic or disciplined enough to realise that.
Anything of which the rules can be modified, is by definition centralized because a central entity could decide upon the modification, and could get the collusion of a majority over it.
I disagree. A decentralised system like Bitcoin is similar to a democracy in that users can signal support for any changes that they want.
You could look at it like proportional representation except each "party" is a mining pool, and the mining pool have a naturally created threshold for being relevant which is a high enough percentage of the hashrate for people to mine there.
Even in most major PR-based democracies, the country ends up being a collection of just a few different parties with different proposals, often claiming to have the same goals while having different ways to get there. Eventually either a majority Parliament is formed in these democracies, or there are problems which eventually have to result in a coalition (a compromise or a chain split in Bitcoin's case).
Bitcoin is very comparable to these democracies and can therefore change while still, in theory, being decentralised.
If bitcoin was truly decentralized, we would not have the scaling issue. 1 bitcoin should equal 1 vote not hashing power.