So, first, the key is not to continue to work long hours and then feel bad if you find it wasn't worth it. The key is to find a life that you are happy with, that suitably balances present and future needs, so that if you do get an inheritance, it's just a bonus.
The other reason to stay Mustachian despite the likely inheritance is that that's a fairly Mustachian inheritance. First, I would assume some of it will go away -- if it's mostly tied up in a house, the market could drop, some of it will be eaten up by healthcare/end-of-life stuff, etc. So say, reasonably, that 15 years from now you inherit $1M. Awesome! That will provide about $40K/yr -- easily sufficient for a Mustachian to live on. Therefore, if you have stayed Mustachian, you are now FIRE and can go do whatever the hell you want for the rest of your life.
OTOH, if you spent 15 years of lifestyle creep because you knew this inheritance was out there, now your expenses are probably $60K, between the cleaners, the vacations, the extra things for the kids, etc. Oops -- now your inheritance only covers 2/3 of your lifestyle. And worse: since you cut back work so much (because, hey, I'm getting an inheritance -- I don't need to save!), you weren't really able to save much for retirement. So now you need to keep working for another 10 years to cover the gap. In this scenario, the inheritance didn't free you -- it in fact enslaved you to work for even longer, solely because you knew it was there and allowed that knowledge to affect your decisions!