The real protection here is that a simple majority can't throw out the minorities voice entirely. But that doesn't require the separation of the executive to achieve, it merely requires either a supermajority, and/or a differently composed senate (which doesn't need to be unrepresentative either, the differing election schedule would suffice, and it doesn't even need a veto - an election-cycle delay would be enough).

Incidentally, from memory, all sliding-into-dictatorship democracies I can think of did so behind a strong "executive". Empirically at least that would suggest that simply not having such a strong executive sounds like a sound protection against despotism. Intuitively, that would suggest that it's pretty hard to rally a country behind a dictatorship that originates in congress; a powerful figurehead matters. Finally, I'm not really sure to what extent a constitution really can or should protect against popular despotism - at best it can delay the process; but part of the point of democracy is self-rule; and taking away that rule to avoid despotism also takes it away for other reform - and it's not really going to work anyhow, because if enough people want to dump the laws, they will, even if the letter of the law begs to differ.