I'm also an only, parents exact same age as yours. We moved a significant distance a year or so to be closer to them, and I'm really glad we did- currently we're a 15/20 min drive, we were seven hours away.
Difference being, DH and I are both from here, though we'd lived and worked in London for years, his parents are from here, and we can both work here (though jobs not as plentiful or well paid). We had already agreed to move back in the next five years, basically before any children started school (currently TTC) but an unexpectedly good job came up for me so we went early.
In the first month we were back, my mum was diagnosed with cancer. The whole process- the treatments, being able to go see her specialist with her, being able to check my dad was ok-was not something I could have coped with at a distance. Initially, it seemed the prognosis was bad, and I really did fall to pieces (a bad prognosis with her type of cancer could have meant losing her within a year). She has recovered, but is generally a bit weaker. They've both aged quite a bit through the shock of it, and there were a few scares which involved travelling to A and E, which at the time I was able to do in ten minutes.
I'd assume your parents have quite a number of years in good health, and don't mean to scare you, but it's worth considering what you'd do in a worst case scenario. There's not a big difference between 2hrs and 2h45- if, say, you wanted to go to a doctors appointment you could still take a day off, but you couldn't do it in an afternoon. You can't 'pop in'.
Is there direct public transport? Are they able to come to you currently? At the moment, your day-to-day would be improved if you moved further away- presume you visit your parents x times a year. How many times would you want to visit if one had a stroke, or a sudden health need? Is there any scenario where they would want to move closer to you? If you move now, then want to be somewhere with a granny flat in ten years time, could you move again easily?
My MIL is in her sixties and calls into her own mother, who is ninety and living independently, every day. It is entirely possible your parents won't need any additional support for twenty years. However I was shocked by how much I needed to be close to my mother when she got sick- I'm normally v pragmatic, but I just couldn't have coped with the distance. My main lesson from that is to seriously think through a whole list of worst case scenarios, what you'd want/be able to do in each one, if the new location will change their impact much and what their likelihood is. In reality, I can't see an extra 45 min making that much difference in terms of how likely you are to visit, or how much time it will take. It will still be a day trip. But there may be other considerations.