ETh is one of the first alts that took off, thats why it will continue to do well.
I don't think it's 'just' first mover advantage. Ethereum has an extremely talented development team delivering constant improvements and innovations. ETH is moving with the times still.
ETH "took off" because of it's utility. It will continue to do well because of it's utility. Utility is what gives Eth value, not luck, not first mover advantage, it wasn't even close to being the first mover.
Eth is the most well concieved and executed cryptocurrency to date. As Ethereum' technology continue to grow with PoS, Sharding and equally as imporyant, developers tools and community, it will become harder and harder to displace.