I think many may not like the idea of flying on the MAX but most simply will not know/care to check the aircraft type before their flight. Many will only realise once onboard (even if then!), especially on the likes of TUI. That being said, TUI is definitely the most prominent operator of the MAX in the UK and perhaps Europe, well at least until Ryanair start flying theirs.
I am booked on a TUI flight next summer that theoretically should be on a MAX but of course I can't be sure. There is a chance it won't even be back by then and if it is, TUI may not be operating all/any of theirs.