I'd let it happen, if the conditions were right. Being stuck in one room actually sounds great to me - if I didn't have things to do throughout the day, I'd be laying in bed with a fridge within reach, watching tv and playing video games.
I know there are giant health risks, and that's something I'll deal with as they come. Choosing to be immobile also means you're not living to 70, obviously, but I'd rather live how I've always dreamed and die younger then to live with major regrets.
I have no idea why it's something that I think about all the time, but being so fat that I can't move my body is a huge life goal for me. The dependence, the struggle to do anything (except eat) in a normal way, and the idea of being kept and fattened bigger even at that size - It's the the kind of life I have spent my entire life aiming for.