Actually it does mean exactly that. What we should be investing in is material technologies which are needed to make solar even marginally functional, as well as better technology for electric grids. That means better and more efficient transformers, wire materials and so on. These are necessities for things like solar to actually work in addition to panel technology itself.
Investing in solar before these problems are solved is like investing in internal combustion engine technology before metallurgy necessary to handle the pressure involved is invented.
Finally in medium to long term, coal (and other combustibles) is in fact sustainable. It's the only sustainable base power we have at the moment in places where there is no chance for hydro and nuclear is off the table for political or geographical/geological reasons. We have more then enough of these fuels for hundreds of years. Coal alone, probably a half millenium at the very least. Then there's natural gas, biofuels and other forms of combustibles. Problem isn't sustainability, it's the pollution. Most specifically CO2 and other greenhouse gasses, as we have mostly eliminated particle, NOx and SO2 emissions which where the actual "pollutants" in the proper meaning of the word.
Essentially coal and other base power forms will remain a necessity until energy transfer and energy storage technologies progress to the point where we don't have to run power plants connected to the grid just because various unreliable renewables like solar and wind might suddenly stop feeding electricity into the grid, collapsing the entire grid. That is what needs to be "heavily invested in" before renewables like solar will have any chance of becoming "ready for the masses", no matter how efficient the technology of those renewables becomes.
Essentially you're making one of the more glaring layman errors. You're putting the cart before the horse.