Downvote me all you want but I'd buy $15 of Bitcoin then wait a few years. So far I have been immensely happy with every single one of my BTC purchases and I don't think that's going to change any time soon. Whatever amount I buy now will be the same % of the total supply (21 million coins) in 5 or 10 years time which is more than I can say about pretty much anything else.
Bitcoin isn’t destined to go up because supply is theoretically set. Demand could easily disappear and there could be a massive sell-off that puts more supply out there. It’s easy to imagine BTC mining being banned in a major country because it’s incompatible with carbon emission goals, raises electricity bills, puts pressure of electricity grids, facilitates money laundering, or any number of things. That would trigger a sell-off and unless some whale is capable of being the unofficial central bank, there’s nothing to stop it.