On the first point, I agree there's nothing wrong with that depending on how it's done.
The problem us that users are expected to suddenly get used to a new UI, and if MS had their way, would have no choice. Any choice we have is thanks to third-party developers recognising what users want, and for large numbers, that is to not have to faff around with MUI.
The reason why it can be good to have a different UI on different devices is that defferent devices are used for different things, in different ways.
The reason, for example, that a supermarket like Tesco uses mouse/kb for their desktop PCs in accounts departments, legal, secretarial functions, and so many office functions is because they work best with accounts software, WP, spreadsheets etc, yet use touchscreens on checkouts, largely driven by "tiles" is that one type of UI suits one type of use, and a different approach suits checkouts.
Exactly the same applies to tablets and phones, compared to desktop PCs. People use tablets in a slightly different way, and for somewhat different things, than they use desktops. And even if MS has a consistent interface doesn't get round the issue that Android and Apple dominate phones and tablets, while MS dominate desktops.
So, in the desire to try to tap into that tablet/phone market, having managed a marked lack of sparkle in previous attempts, MS decided to force existing users to do things differently, regardless of the hassle or aggravation of doing so.
Even so, forcing a change might be a decent decision IF the change you forced was a better way of doing things, but the MUI paradigm does not, in the view of lots of people, suit conventional desktop usage.
One reason the MS attempt to shove MUI on us is precisely that 3rd party devekopers have offered an alternative. MS could have, and in my opinion, should have, offered that from the get-go.
Here's an option.
As part of the install process, MS could have offered the user a choice. Option 1 = the new, "improved" MS MUI, and option 2 = installing a third-party of MS tool to maintain boot to desktop, start button, etc.
After all, MS have incorporated enough 3rd-party tools in the past, like disk defraggers, backup tools, etc. And it could even have been "unsupported", a la Powertoys, provided by but not supported by MS.
They could have offered users the choice to engage with a change that requires users to get used to a new way of doing things, and in the case of business users, very significant retraining costs, or not. But they didn't. So, I'm forced to assume that MS are either so thick that they didn't think about it, or that they decided their long-term strategy trumps user choice and preference. I know which of those my money would be on.