It is certainly possible to break Linux systems with upgrades. I know that very well from the time i supported sid. In most cases you can fix those issues, the most generic way of "fixing" d-u problems is repeating
until you don't get any error. thats the basic variant, however when a dist-upgrade wants to remove core packages then something really bad is going on and it should be avoided to try at all. In theory it is possible to reinstall removed packages but thats not always that simple. If just minor packages have been removed, like gimp was removed because only gimp-data was there for the used arch you can install gimp when both packages are available. As i do not use Ubuntu - i only use some interesting parts like kernel (with some mods) or mainline builds, linux-firmware packages. Sometimes i install U to test my fglrx script, but i dont keep that install.
apt-get install -f
You are definitely right that you can not see Linux as a Win replacement for the 08/15 user. If a possible Linux user just surfs the web and was attacked by a virus/trojan then this one might switch. Usually you can not convert pro-gamers, thats more or less impossible, basically you can suggest using live systems for Internet usage but thats all. For ppl which specific win app needs it can be enough to provide a win vm - as long as those apps dont need 3d support. Sometimes wine is already enough - wine improved a lot, you can even run Office 2007 without problems. Of course there are much more Linux users out there (or dual boot users) than years ago, Linux marketing helped there. But of couse there will be always ppl who have got the wrong ideas what to expect from the system and are disappointed then.
I do not think that the new W8 UI will lead more Linux users. Some might just install W7 and praise the old times but those do not switch to Linux. I mainly test the technical aspects, learn a few new shortcuts and want to be prepared. It is always good to have deeper knowledge of a system, it does not matter if it is called Linux or Win, just that Win becomes much sooner boring as you can not modify/tune it the same way as Linux. Of course for unskilled users every system looks complicated thats not the same as usual...