In laws might move close to us, how do we tell them not to assume we'll be here forever?

My wife and I, both 30, recently married and bought our first home together. She has two siblings but nothing indicates they will ever have kids/marry, if you met them you'd know what I mean...

Her parents currently live in another region that is either an 11 hour drive or flight away from us. They now visit about 1-2 times a year and we see them about 1-2 times together and usually my wife will make one solo trip out there a year. She also calls them to check in every weekend. Despite not seeing each other a ton, they are pretty close.

They clearly see us as their only path to grandkids so are now talking about finding a retirement home in our city. Im not thrilled about the idea of them moving here, but I recognize they are adults who can do what they want. They are not my favorite people to spend time with, but they are objectively nice people. We just have very different backgrounds and her mother tends to say some judgy comments that get under my skin. Her dad is nice but just kind of annoying, not a bad guy though.

Here's the thing though, the retirement homes they are looking at cost close to 1m to buy in, but guarantee their spot for life. They are well off enough that they could afford this, but it does have a pretty big penalty if they ever did leave. However her mom is in her mid 60s and relatively healthy, her dad is in his 70s and slowing down a little but not on the verge of death or anything (seemingly). They could easily live here another 20-30 years.

My wife and I have lived in our city about 6 years each and love it, and have no plans to leave anytime soon, but we also arent married to the city either. In fact I always assumed Id move a couple more times before its all said and done. I want to make it clear to them that while we dont object to them moving here, them being here doesnt mean we'll never leave. Im not commititng to spending the rest of my 30s and 40s in this city just because they wanted to move to be close to their grandkids.

My wife is in agreement but not sure how to tell them this. What is the most diplomatic way to get this point across?