There's a sort of mixing of units happening here, and I think it's causing some confusion. Here's an example (greatly simplified) scenario highlighting a flaw in your rationale:

1. Energy at your normal usage costs $1000/yr.

2. You can spend $20k now to have access to equivalent energy output for the next 40 years before it degrades to unusability.

3. Next year, somebody invents a flux capacitor bringing all energy costs for everyone down to $1/yr.

If you don't buy the thing, you spend $1039 over the next 40 years. If you buy the thing you spend $20k, and it's hit its expected lifespan, so you don't recoup any further benefits.

The real world has inflation, wars, more sane invention deltas, and all sorts of complications, but the general idea still holds. If you expect tech to improve quickly enough and are relying on long-term payoffs, it can absolutely be worth delaying your purchase.

If you predict massive improvements in solar/battery/etc tech, the only way it makes sense to invest now is if those improvements aren't massive enough, you expect sufficiently bad changes to the alternatives, etc. I.e., you're playing the odds about some particular view of how the world will progress, and your argument needs to reflect that. It's not inherently true that just because solar pays off now it will in the future.