I invent the toaster and sell the patent to Cuisinart. They decide not to produce it and sit on it for 15 years. Then the patent expires and anyone can make toasters. Now Cuisinart has lost out on growing a reputation for making toasters. That's their loss and the public benefits because they have toaster technology. And, the original inventor was rewarded and can go on to invent more things or inspire others to invest time in inventing because it pays off.

You're not thinking widely enough. The following is not the real story, just an hypothetical example:

You're Codake, a company that sells chemical photography materials.

Some person invents digital photography, you find out and buy the patent from them for 500K.

Digital photography would hurt your profits so you just sit on it for the 15 years, maybe start designing some products around it in the last 3 years of the patent to be ready for when it expires.

You made more money and society lost out for 15 years, all because the inventor probably wasn't used to seeing large checks and didn't shop around when the largest photography company offered them more money than they ever saw in their life.

Even the inventor got screwed, by selling the patent they got a nice house even though it was probably worth hundreds of millions.

I dislike patents but there's also the element of risk. Most inventions flop.

Yes, the above scenario is quite risky. And if the inventor "got screwed" then that's on them for agreeing to sell for a price that's less than the value of the invention. That said, execution is also very important. Ideas don't build companies.

Personally, I wonder if your argument changes if the thing in question is not a toaster but something the public wouldn't want to wait 15 years for. Medical vaccine, groundbreaking software, or an improved biological process using less antibiotics. A toaster doesn't really have immediacy, but this policy affects things that do.

I don't have a solution, just a vague feeling that in 2021 15 years seems like a very long time (for software, at least.)

What company would buy the IP for a vaccine and not produce it? They cost billions to fund. It isn't sustainable to buy and sit on patents unless there are judges willing to uphold invalid patents for patent trolls, which is what has been happening with software patents.