It's not you guys personally, I am talking in general. As a product company, you (the company) should be aware of such problems. Not saying they don't care, because I can't give such a statement. And I think you mentioned that moderators may notify them about important things in the forum. So I hope they will be aware of this one.
Back to the problem - the systems I use are not "20 years old". They are not even 10 years old. I know that the software world and technologies are developing fast. Yet again, what is the reason to change something that worked before for everyone? I don't see any performance increase in the fights. But I do see speed up when open the Harem (yet still there is a problem with a missing girl when scroll down). I see the reason of using .webp instead of .jpg/.png.
BTW the "forcing update" is made more often to have all (or at least the majority) of your users to use the same version of an app in order to avoid supporting older versions with old technologies. In this case, we are talking about a web based app, which do not have that complex logic, thus you do not have to force players to update their browsers. It can be made to work again, as it was working before. Sure, there are unsupported browsers and there is a message about it when load the game. In case you see such message, you can expect problems.
Lastly - I don't think this is connected to the Win XP or Win7. It's only a browser/script thing.