75% is generous, normally you need 67% to change constitution laws in most countries. It is always tradeoff how big minority you want give the right to veto majority will. If you think 5% or 10% can veto 90% majoriy, your probably part of this minority and want more power than you actually have.
Is Bitcoin a country; a democracy? You're comparing apples and oranges here. Bitcoin is a consensus driven protocol. Participants that started using it have agreed on a set of rules.
What about if we need 90% support for not increasing to 2MB, otherwise we increase to 2MB. You see, just 10% could veto such change even if almost 90% would disagree. It is not right to give such small minority much veto power in eighter case in my opinion, 67% mostly used has reasons behind, and 75% is very generous imo if 26% still can be allowed to veto 74% will
Wrong. If 2 MB blocks are "urgent", "needed right now", "wanted by everyone" there won't be a 5% to oppose it, not to mention 10%. If there is, then those 3 are invalidated. 75% is not generous, it is harmful to the network and all the participants. A hard fork should be an upgrade of the existing chain, not a split into 3/4 and 1/4 pieces.