Where could I store my assets in an emergency if there are no more bitcoins?
Before thinking about plan B, I think you should focus more on Plan A which is buying more bitcoins now while you can. Bitcoin will always be in circulation because in the future, more people will be using bitcoin as a payment option.
The OP thinks if Bitcoin cease to exist that other alternative cryptocurency will continue to exist. Without Bitcoin there'll be no cryptocurency market as everyone would had lost trust in the market. Bitcoin is the reason why the market is still functioning and very popular today.
Other projects look up to becoming as successful as Bitcoin has become so if Bitcoin cease to exist, you might as well take your money out of the cryptocurency market as it will collapse heavily as tokens will be dumped. If there's an emergency store your more in stocks.
I don't think any other cryptocurency comes close to Bitcoin. It's Bitcoin or nothing else. Other altcoins can be easily manipulated and if anything was to happen to Bitcoin, whales will take that opportunity to manipulate the market and buying other coins put you at risk of loses.