1) Leave the possibility to change css is a problem, because someone must PROGRAM the function inside the source code.
2) Changing GUI is a problem for assistance, because are introduced all the problem of all user changing GUI and cut himself out for make useless interface
because they are not designers and do not know css
3) Better Windows 98 are more nice and clear than Windows 10.
4) WinBox is browser indipendant and have same aspect on all platform
1) Yeah, you're right. I am not the developer of routeros but I don't see linking uploaded CSS file into webfig as big deal. If it's hard to implement, then there's always option to just update current webfig css to look more modern. Same UX, layout, just new UI. I believe that such a big company as Mikrotik has at least one designer.
2) I don't think that unexperienced users would be able to change css, moreover the way they would destroy something. Manuals and help knowledge use default skin. Assistance worker can simply ask user to use it either.
3) That's individual feeling and that's ok. Others prefer different design.
4) Then why couldn't webfig look different when there's always option to use winbox? Unexperienced users usually don't need to use winbox (because that's simply another app they need to download to setup router) and webfig is first thing they see.