My view on this sits somewhere in the middle of both extremes of the subject. Ultimately, I feel if the hotel does not have a strict dress code, I can wear whatever I want, within reason, e.g. stay clear of the mankini. The fact is, that what I want to wear, is what 90% of people would consider acceptable wear, basically jeans, trainers and a jumper/hoody. I don't feel I'm offending anyone by doing this and feel it's up to the hotel to decide the dress code, not the guests. If enough guests express displeasure are other guests staying there, management will find out and will have to enforce these standards. For example, if I were to go on the Venice-Simplon, I would wear jeans, unless it was not allowed. This is my choice, as it is the choice of companies to not allow me there if they so choose; I don't see why I should have to spend so much, so I have to spend the entire time ensuring everyone else is completely satisfied with my appearance.

Even 2 and 3 Michelin-starred restaurants have reduced their dress code significantly in the last few years. Places like Midsummer House and The Fat Duck don't even have one anymore and even Royal Hospital Road has lowered the standards required.
I will not visit The Ritz Restaurant (although I have stayed in the hotel once), as they insist on wearing a tie and I don't want to ever wear one. I won't complain about it, or ask if they can make an exception for me, I simply accept the rules and won't ever attend unless they change it.