Bitcoin is a store of value and a medium of exchange, just like any other currencies around the world. The problem is that almost nobody wants to use BTC as a medium of exchange, because everyone expects the price to go up in the future. Bitcoin might be dethroned by a better altcoin, that has cheaper and faster transactions(assuming that this altcoin will be secure and truly decentralized). That doesn't mean that the function of BTC will change.
By dethrone you mean a bitcoin stepping down as the number one coin, seriously you might be right that bitcoin getting a competitor from an altcoin as a faster transaction coin could make the coin be favourite but i would say no, no coin currently can over power bitcoin because of one thing which is total decentralization and having that speed transaction confirmation, the both just couldn’t happen together if not we would have seen it emerged. The one that comes close In having both qualities is monero, in fact monero seems to be almost more decentralized than bitcoin and what happens to its emergence? Government and exchanges treat it as a threat, that’s why others even in future wouldn’t flourish too.
"Faster, cheaper" coins have literally been tried hundreds (maybe thousands) of times and they have all failed. There is a reason Bitcoin is valuable and its not fast cheap transactions, which it can do anyway with LN.
Bitcoin is already the savings/store of value currency of the present and future. It's won that already. Nothing comes close. It still has to develop that use case on a global scale but it is clear it is on the way. Once that is established then, and only then, will it make sense for people to start using Bitcoin as a medium of exchange. There is no other cryptocurrency that could possibly compete with Bitcoin as a medium of exchange because to get to that point your need global mass ownership, and the only cryptocurrency for which their is a clear value proposition of long term ownership is Bitcoin (and maybe Ethereum but that is not a currency but just an app network base token and one that is not very decentralized).