Bitcoin is already being outdated by other cryptocurrencies, even Ethereum was a generational improvement on Bitcoin. However, as we all know, the best isn't always the most successful, and bitcoin is clearly the most successful. However, modern cryptos such as ADA and EOS could possibly overthrow bitcoin if they manage to gain significant traction and partnerships, this is because they have much more potential outside of just being a payment system.
I think if the trend of bitcoin price is going down for the next years to come, indeed it will be outdated by the other cryptos. People will eventually put their interest in other cryptos that are really beneficial to them. Bitcoin might be on the verge of loosing its dominance if the price goes very low while the others are making their way to the top slowly but surely. Just my opinion anyway, that might shed light.
It is hard to believe mate how easily you said people will create some other currency, is it a child play to make currency and allow it to the markets as payment, no it is not going to happen already bitcoin is in the ground and it will be there forever, no currency can ever replace bitcoin, it has gained the most useful most invested currency award so I think bitcoin is hard to beat by any other currency,