Make no mistake about it, the closest competitor fro ETH is NEO. Other potentials are EOS and ICON. But if I had to say could ETH be overtaken within a couple of years by anyone, then I would say NEO, for all the reasons already mentioned in this thread.
For me, the best competitors in ethereum is bitcoin because this is one we promote as a single currency due to higher demand and it explodes directly its prices. Instead of that in the part of, altcoin I think ripple is one of the best coin also .
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There are too many altcoins that are good in the market and have a good technology, and even though the market value is not too prominent I think it is not impossible that they will appear and become popular in the next few months.