The HSS guitar is not a good idea
Is there a solution to this problem?
Yes and no.
As I said above, almost all electric HSS guitars have this problem. One doesn't. Sort of.
At the time I write this, there is the new Fender American Ultra Stratocaster HSS. Part of the selling point of this guitar is that Fender did solve the "volume drops off a cliff" and treble response issues with some cleverly engineered electronics...
...but there are other solutions. Or rather just one solution.
The pickup configuration where a humbucker and single-coil can mostly live happily together is when you combine humbucker + P90. But this is all dependent on whether you actually like the P90 pickup sound.
Before I get into that, I'll explain the P90 in simple terms. Compared to the Stratocaster single-coil pickup, the P90 is physically larger, has greater output, less treble response and more midrange response. It's also usually mated to a 300K volume pot, but can still sound decent with mated to a 500K.
P90 does not sound like a Strat single-coil but still has single-coil character. Think of it as the middle ground between a Strat single and a humbucker.
I personally think P90 pickups sound great, but many players who try them do so on the expectation it's going to have Stratocaster or Telecaster sound character. Nope. P90 is P90 and you'd have to hear one for yourself to understand that fully.
The point I'm making is that if you have a guitar with a humbucker and a P90 mated to a 500K volume pot instead of a humbucker and two singles, that to most players would be a much more agreeable sound to the ear overall.
The better idea however is to keep singles and humbuckers in separate guitars. It's not a sin to own two guitars to get two distinct sounds. If that's what you have to do, that's a-okay.