they could join the L-mount

They "could" but who says Leica would approve that.They have the final say.

L mount doesnt need anyone to join,its not open like M43.

Speculation and wishful thinking is one thing but reality is another.

This is only my opinion, no one has to agree with it, but trust me I am an engineer.

I think the future is mirrorless, it just has better properties then the DSLRs, we just can't use them all now with current tech.
The mirrorless has at least these advantages:

• No need for separate AF and AE sensors, the main sensor can do all of this.
• No need for AF adjustment, the main sensor always can check it with contrast detection.
• It's lighter, the prism is big and heavy.
• Shorter flange distances are possible, so more tech can be added in place of the mirror or thinner body can be created.
• Shutter lag can be totally eliminated. If the (future) sensor captures continuously, then the camera can save the image taken when you pushed the button or even before you pushed the shutter button.
• Any custom overlay can be added to the viewfinder image. Focus peaking, histogram, under/overexposure warnings...
• Exposeur can be simulated. (I like it when I half press the shutter button)
• Low light manual focus is easier with "correct exposure" viewfinder image

Thanks to the VR madness small high quality, low latency, HDR monitors are advancing very quickly, so the viewfinders gets better while the research costs goes to other segments, because these are mandatory for VR headsets. I think EVFs will catch up in the next few years.

While I'am a Pentax shooter and I really like my KP and the OVF, I think the future is the mirrorless with EVF, we are only waiting for faster readout speeds and lower power consumption. (Lower consumption is also mandatory for the sensor cooling, we need a continuously running FF sensor for these!)

So where is the mirrorless future proof Pentax camera (or at least development)? I think there are some technologies they are trying: The K-70 has on sensor PDAF, so we seen the first version and I think the LiveView is good in the KP.
Altough Pentax does not have their own sensor development like Canon and Nikon has according to the fresh news, they really depending on Sony and we don't know how many alternatives are there. Sony may be prohibiting for Pentax to make mirorrless in change for the access to the relative new sensors. I trust in the engineers at Ricoh, they have a much more information about the market and the opportunities, so I hope they choose the right tactics.
Maybe we will see some tech in the GRIII which confirmed to be coming.

Of course there is another factor: the K mount which keeps many of us at Pentax. A mirrorless needs a new mount, with new technology. While there is the option to make "designed for mirrorless" KAF4 lenses, I think we would loose too much by keeping the mount. Although getting to a new mount would split the not so fast lens development team too, so I think they currently does not have the resources for a mirrorless system.

.... Nikon's mirrorless doesn't take off, killing Nikon. Pentax decides not to risk mirrorless. Canon, after several tries, develops a good mirrorless system....

I came here after owning a Nikon 1 J5 mirrorless

some of the exposure settings were not easy to use.

They probably lost a lot of money making a new set of lenses for the Nikon 1 series.

the EVF was a real problem in bright light (most recently, when snowshoeing in the sun, I would just point it best I could for the picture, then put it under my coat to look at the result)

I think the J5 did not have an EVF like the V1 had, only the screen on the back? The V1 had an EVF and with that I had no problem at all in bright light.

But whatever, I only had the LCD screen.