Consensus? What does that mean? Hashpower? Non-mining nodes? Wallets? Percentage of BTC in existence (hodlers)? What arbitrary number would you like, if 75% is too low?
That means everyone. Miners, services, users, etc. You can't do this quickly and you can't do this with 75%. Essentially in order for a hard fork to be an upgrade the threshold has to be so high that the old chain has no chance of living else you're splitting the network in two. I'd say 95% being a minimum. Also, anyone arguing that a single entity could prevent the hard fork because of the threshold, the same can be said with 75% with just a small group being against it.
Your attack on Peter_R notwithstanding, I don't believe the absence
of a blocksize limit is necessarily a fatal flaw. You don't have proof that it is,
and to believe you know with certainty it is, is surely closed-minded.
It is either fatal or damaging. There's nothing positive about it.
^ See this is what I'm talking about.
A fee market is just a means to an end
(an incentive for participation in PoW).
The more decentralized and accessible the
mining ecosystem is, the more you'll get
natural participation and security.
You're also making certain assumptions,
such as that miners would behave in
a way that would undermine the natural
fee market based on orphaning risk.
You also make the assumption that
it would be impossible to get users
to pay fees voluntarily, perhaps through
default settings in clients. If Bitcoin
reached 200 tps and everyone paid
a nickel, that would be $864,000 a day
in fee revenue. I'm not saying this
should be relied upon, just that in general
there are many ways to bolster network
security if one is willing to be creative
and get out of dogmatic thinking that
things can only be a certain way.