Some people would pay slightly more for a house that already has solar installed, even if it’s a 5-7 year old system. So install it, then use it as a selling point down the track.

Anecdotally, I’ve rented a house before that had solar installed. It was a factor in my choosing to rent that place. I don’t know how it is elsewhere in the world, but it’s pretty common in Australia to have to pay for your own electricity usage when renting a place. The solar panels lowered my bills considerably.

Pad mount / ground mount systems are a thing, if you have enough space. That said, anything that can be installed, can be removed. Cost is a factor, though.

I don’t see any breakthroughs coming any time soon. Gradual improvements through economies of scale will likely be it, along with any government incentives.