Well the only problem I would see in buying it is that it's possible Apple could make it free in the future or even cheaper. However I just checked the website and before, it looked as if though the company was winding down as they were being absorbed, now it appears as if thought they are fully updating it, even adding new user guides. The front page doesn't even mention anything about being owned by Apple, they just explain how well integrated they are into macOS as they always have.
It is a fantastic app though and no matter what it will still be very useful in the future. I'd argue it is probably one of the best apps you can even get on your Mac. Besides it possibly going free, the only other thing I would worry about is maybe the app becoming universal, and therefor making the Pixelmator on iOS, pretty much free. But yeah I definitely think you should buy that app you will enjoy it throughly.