I doubt anyone actually believes Microsoft considers the "supported lifetime of your device" to be only a year or two for a desktop computer.
True, but people would have said the same about Apple once upon a time, while lately Apple's software policies seem tailor-made to artificially limit the lifetime of its already relatively expensive product range, up to and including the high-end business laptops and such.
I think the concern is that this is a one-way trip. Once consumers and particularly businesses start making the switch to Windows 10, it is unlikely there will be any going back.
If Microsoft then ships one box-bricking Windows update to all those Windows Home users, who will have no option to defer or skip any update under the current proposals, there is going to be carnage.
The other significant risk I can see is that if Microsoft's new business model doesn't work out -- after all, it seems they're essentially betting on giving away Windows for a considerable time in the hope that it will drive more sales of other software, media content, and related services -- then they are going to need to make their money somewhere else. It would be a brave person who bet against a major tech company exploiting its locked-in users in the face of shareholder anger and probably changes in senior management under those conditions.