I'd be very interested in a Windows 10 phone if it has automatic updates that do not depend on the carrier, to avoid the Android situation where only certain phones get updates on a timely basis.
Microsoft has chosen to drop a lot of older models from the Creator's Update, but is still providing cumulative/security updates to the Anniversary Update right now, which is supported on all phones which run Windows 10 Mobile.
At least, at present, if you have a Windows 10 Mobile device, you have recent security updates, regardless of make or model.
I would say, Microsoft has a reputation for long term support and backwards compatibility, but it seems like it's something they're trying to get away from.
The manual intervention to upgrade to 10 is because, as noted, the big change is that Windows 10 Mobile upgrades are handled by Microsoft, so the update servers had to be changed. Prior versions to 10 had the same carrier-based update issues Android had.
I'd argue that change alone points to the fact that Microsoft is getting better at long term support, not worse.
No, that's a good thing. Pushing major updates and new features instead of just patches is what makes users (and IT departments) disable automatic updates.
(Although I'm in the UK, where the carriers are less abusive)
Worse yet if the update fails...
Any phone above 300€ is not worth my money, I am not buying a gaming desktop.