New technological and manufacturing advances happen nearly daily. We may have panels that last for 60-90 years within a decade, meaning that costs will drop even further.
Solar also has the advantage of being extremely scalable and decentralized, so that a massive installation can be put in cheap land far from any people, without having to run transmission lines to a big centralized power generator. Of the current average cost of $0.13/kWh in the US for electricity, $0.08/kWh is for transmission and distribution costs, and only $0.05 is for power generation. That 8 cents isn't falling in cost, and so halving the cost of generation, or even making it a fifth of the cost, doesn't help a lot unless the power generation can be located closer to where it's needed.
For super cheap energy, it's going to be nearly impossible to beat solar PV.