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... software, that's the interesting part.

But it's so hard to quantify that the question is rarely explored.

(Also, let's assume the awesome new version *also* grows organically, so the "lifetime" must be capped at "while it is still a reasonably clean code base", which makes the problem even harder to grasp).

The TL;DR is, we won't get many new browser engines, not because it's impossible, but because people with control over purse strings rarely look a few decades into the future.