So the cryptocurrency's first generation is BTC (1.0) and second generation is Ethereum (2.0).
Blockchain 1.0 addresses the first generation as Bitcoins and forks. Blockchain 2.0 looks at extended functionalities.
What will Bitcoin do to stay relevant in Blockchain 3.0.. 4.0... 5.0 ?

Bitcoin will stay in any generations as long as bitcoin is the foundation of most exchange bitcoin will remain strong.
Yes, bitcoin is the first generation of cryptocurrencies and we can't ignore that thing that this coin is the basis of all price of any coins. Yes, Ethereum came and it is being used in exchange too but most and major exchange are into bitcoin and we can't change the fact that as long as this way stays bitcoin will be on any generations.