I don't think it's a waste of money as long as you are still able to find items that you like for it.
Creators do tend to keep up with the latest developments. For example, Lelutka's upgrade/change to custom UV maps for their heads and ears to increase their realism and quality. This will introduce a new type of skin and ear customization to the market as creators make products for this new line. The market seems to be moving in this direction as well (which is also suggested by the Catwa HDPro, as you mentioned).
The thing about using an older head is that (from my assumption) the UV maps are not as updated and therefore, cannot achieve the HD look.
So if you're looking to increase the quality of your avatar, an upgrade would be the way to go.
That being said, the applier system for makeup and skins still remains, as well as BOM. I'm pretty sure the Catwa Catya head can still use appliers and BOM, so you don't have to worry about that.
I think the issue is not necessarily about your head becoming obsolete, but rather the kind of look. There are still users who prefer the Classic avatar and have no issue, but since mesh is what is mainstream, that look is considered outdated.
It all comes down to what kind of changes, if any, you want to make to your avatar.