There are two main issues.
1. A small, but not insignificant, number of upgrades go south. I've not had problems with machines that ship with 10, but I've had fix more upgrades than I would've expected.
2. Respect. Show some Microsoft. I, my company, or clients paid good money for the existing computers and we are relying on them to get things done. Nag a little - that's OK. But what [blankty] [blank] [blank] you [blankers] at some point Repect the fact that I (or the client) have chosen to decline the free up grade for a good reason - or bad - and the problems with having an old OS are on me. It could be inertia, liking the look of Win7, or it could be an office full of machines that have to connect to an application that requires IE-10 or lower. I might already be [blanked] off at the application vendor who still has no timetable for getting off IE-10 when you decide to it's a fine time to nag me YET AGAIN while getting sneaker and more insistent, making hate not so much Win10 as Win10Upgrade badgering ...and leaving me no choice but to go rant on el reg.