What are your thoughts on taking old, VC tested/failed ideas and trying them again? The only difference in this new scenario is that instead of taking outside funding, bootstrap the company and grow at a sustainable pace.
Given that, yeah, I think "failed" ideas are a gold-mine of potential. The big question, of course, is "can you identify the ones that are fundamentally good, and just need a little polish or maybe a tweak to the business model, from the ones that are just plain wrong?" And if so, can you come up with the little extra mojo that would allow that idea to be brought back, and become successful? If so, then you are probably onto something.
FWIW, I spend a fair amount of time looking at old ideas (not specifically ones involving failed VC backed companies though), always thinking in terms of "is there something here that can be re-invented, re-mixed, re-purposed, etc?" I can't claim any major successes yet, but I have a few things "in my hip pocket" that I hope to get too eventually.
Anyway, I say "go for it."
Some ideas don't get traction because they are before their time, i.e. technology might not have been up to scratch. For example some apps might be better suited to mobile than desktop. Or need current generation browser capabilities rather than those of 3-5 years ago. A good example is apps that relied on Flash or Java applets can now be implemented using HTML5 + CSS3 + ES6.
Another common issue is needing more internet bandwidth or lower latency than was available at the time. There still are many areas where internet speeds are far lower than those in major cities or suffer slowdowns when congestion increases, e.g. evenings.
Then some ideas didn't get off the ground because the market wasn't / isn't there. Those ideas might still be a very bad bet.
And ... considering your bootstrapping ideas, many ideas just don't scale to 100's of millions, but can result in a 100's of thousands or even a couple of lazy millions. Markets of that size simple do not appeal to VCs.