If anyone feels Pentax is such an awful system consider why are you supporting it by buying their cameras in preference to other marques? Nothing's basically changed in the last thirty years, or make that 50 if just considering the K mount. Other marques can't boast that.

I've had someone print a 24x16 from a cropped Pentax Q shot. They were quite happy with the result.
Bear in mind that lower pixel density means less noise in the final result which is why FF and MF don't require it increasing, it's really good as it is.

I'm thinking most Pentax users are buying Pentax lenses which means Sigma have a lot of K mount lenses to get rid of and they have no wish to make matters worse. I'd feel the same if I was Sigma. Buy them or loose them.
It is worth considering that Pentax cameras have correction profiles for Pentax lenses but not for Sigma ones. Sigma's 8-16 doesn't even report the wider focal lengths to the camera.
If it's Sigma lenses people want, buy a Sigma DSLR or mirrorless. Problem solved. But would you expect to get Pentax lenses for it or moan if you can't?

I believe those lenses are still available brand new from retailers if you wish to buy them. Do you want links for them?

With Tamron and Pentax working together it might prove to be a life saver for both if the Pentax patents are used in Tamron lenses which get ported to other camera brands.

Tamron are working with Pentax to produce new lenses hence there seems to be an agreement not to duplicate them.
Irix, Laowa and Samyang are releasing new K mount lenses, eg wide angle primes.

Don't forget that any new FF lenses can be used on APS-C too,

No, in your own words Sigma are!
But what if Pentax were to concentrate on FF for a while. Having introduced a FF body they'd be a bit stupid not to support it would they not given there's a plethora of APS-C lenses available already?
I don't get it that we must have something new every week. There are more gaps in the FF range than in APS-C where just about every usable focal length is available.

Well use a more local dealer then :
eg Carmarthen Cameras, Zion Chapel, Mansel Street, Carmarthen SA31 QX
Believe it or not SRS isn't the only Pentax retailer in the UK.

There's a few users reviews over at PF.

I'll provide this link in case you've been tempted that the grass might be greener on the other side of the fence. Would put me off for sure.