Dad - SIL offered to be his paid carer

This gives you an excellent excuse not to dance attendance on your Dad and just keep your visits social. I would leap at this idea if i were you.

OP's life isn't going to be sacrificial to her df's though. The SIL is going to look after him and OP is going to wash her hands of it all.

so why shouldn't he be lazy in his final years?

Because it is not good for him - or for the OP! My father decided he would behave like this a few years ago, now he can barely walk (it really is a case of use it or lose it when you get to that age). He has hugely swollen legs from sitting all the time, which have ulcerated. He frequently gets stuck on the toilet - and only partly because of his physical health, mostly it is due to his refusal to put in any effort, so he will try once, in a half hearted way, then just sit there and wait for my step mother to try and lift him off, which she often can't as she has arthritis in her shoulders. At this point he starts yelling at her to get one of the neighbours or phone me (I live an hour away) to help him wipe his bottom and get up.

OP you are right to be concerned about yourself and your future. My dad used to say he would never expect us (me and my brother) to look after him when he got old, said he would rather go into a home. Now he thinks it is his God given right that we dance to his tune as often as he wants to play it. But elderly people often get selfish. This does not mean your life should be sacrificial to his though.

He's 84, he's old and tired, and he thinks how nice it would be to be looked after. He'll also get company (maybe he's a bit lonely). Realistically he's not going to be around for very much longer anyway, so why shouldn't he be lazy in his final years?

In all honestly, Dad health is very good for his age. He can walk (with a stick) about half a mile. Not bad for 84. Hes just more interested in making no effort at all. Hes my Dad but I find it a little pathetic to be honest.
Perhaps cut your 84 year old dad a bit of slack? Unless there's more to this?

He’s not going to be healthy forever though. It can be quite shocking how quickly older people can deteriorate. It’s probably for the best.

Why are you bothered? If it makes him feel safer isn’t that a good thing?

Honestly, I'm keeping out of it. Washing my hands of it all. That's all you can do. They're adults, they can make their own decisions. Just be pleased that it lets you off the hook.