First, it wasn't "safe" for you to become a millionaire back then. It seems safe compared to now because we KNOW what's going to happen already, so don't be dumb enough to assume that that's always the case. Second, it seemed like "modest" investments now, but with the same logic as the first, it only seemed like it. I mean, would it seem modest to invest thousands of dollars on an unknown currency that popped up all of a sudden without any company backing it?

Any altcoin could potentially give you big profit, the margin of growth in new coins is a lot bigger in comparison to old coins after all, but at the same time, just like when Bitcoin was new, the risk is comparatively higher compared to well-known ones. As for Bitcoin making you a millionaire, who knows? It may be a modest growth, or maybe even none at all. I'd suggest just doing DCA though instead if possible.