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.
Yes. NEO is considered one of the closest competitors for Ethereum to date. However, there are other strong competitors in the market such as Cardano and EOS which aim to provide better features and greater scalability than Ethereum itself. If NEO and other smart contract platforms prove to be faster, cheaper, and flexible than Ethereum, then they would take its place in terms of usage and popularity.
However, given that Ethereum has gathered a lot of attention and support from individuals and businesses alike during the past years, it'll be hard to take down since it has first mover advantage like Bitcoin. Of course, better technologies could emerge which promise to solve the issues of scalability and smart contract efficiency, but I believe that Ethereum will stay on top of alternative smart contract platforms for years to come.
Nevertheless, if we were to list the strongest competitors against Ethereum from the most prominent to the least prominent, they would be: EOS, NEO, Cardano, Qtum, and Lisk. The world of crypto land is full of surprises, so anything could happen in the future, and it's only a matter of time which smart contract proves to be the best by then. Just my opinion