I think YABU. You are happy where you are and that’s a good thing. I presume your parents are happy, though you say they would never move even if they weren’t “ because family matters”. This suggests you view that as the only “right way” to do things. But would you genuinely prefer that your PIL to be hankering after something else so that they can be close to you? Would that not create a lot of pressure on you too? If they stay for you, it’s going to be hard to justify you moving if you ever wanted or needed to.
Your PIL are moving to somewhere they really enjoy being because they can now they are retired. It’s great that they can do this whilst they are (presumably) in good health- they only get one life, just like you, and I think they are trying to make the most of it. I get that you might miss them, but 3 hours really isn’t that far.
They maybe didn’t want to say anything before it was decided and sorted as they knew you and DH would be upset and angry, or would try and talk them out of it. Or perhaps they genuinely didn’t feel the need to consult you both about the situation and/or just assumed you’d support their decision if that was what made them happy etc.
I don’t suppose you’d seek approval from them (or your parents, for that matter) if you wanted to move away- either for better job opportunity or because you found somewhere you loved being or any other reason- and how would you feel if they were angry and upset at you for wanting something different? I don’t see why it is ok for younger people to move if they want (or need) to but it’s wrong/unfair for people who have adult children (and even worse if they have DGC).
I suspect if they didn’t take DGM many would accuse them of “leaving you to pick up the slack” and I think you might not be best pleased to be left with that responsibility either.
FWIW, one set of my grandparents lived about 3.5 hours from us and we only really saw them in school holidays (though they phoned every week or so- pre face-time era!)- we would go up and stay for a week or more, sometimes with and sometimes without our parents. They were wonderful grandparents, we had the best times and I have so many wonderful memories of them. We were far closer to them than my other set of grandparents who we saw very regularly as they were 20 minutes away. I think it was partly a personality thing, but I think because they really valued the time we did get they focussed their time and attention on us when we did see them in a way they might not have wanted to/been able to if we’re around all the time. I can’t say for sure, but I just wanted to say that being close by does not guarantee a good, close relationship and being far away does not mean DGP’s cannot have a good, close and loving relationship with their DGC.
I would also say- as someone whose father is in poor health and has been for many years (since he was mid-50’s, actually- so not too dissimilar in age to your PIL), whose health is fading more rapidly as time passes- it’s fine to miss them, but be grateful for what you have. I would love for my dad to be well enough for him and my mum to go off and have an adventure or decide to move somewhere just because they love it there. Even if it meant I’d see them less, and my children would see them less. I think it’s great that your PIL get to fulfill ambitions and do things they want to, go where they want to and experience new things. It could so easily be taken away from them tomorrow or next week or next month or next year…. I say good on them for making the most of their retirement.