Exactly. ETH IS NOT DECENTRALIZED. Indeed its quite the opposite.
This may not necessarily be its downfall, but it could well be in the long-run. Certainly a huge issue (flaw?) that people need to be aware of. Short term though seems lots of excitement around it, so occasionally we will see eth pumps (and alt pumps) as BTC's overall market cap grows...
It's far from a potential downfall.
Right now bitcoin offers very little other than being a store of value or "digital gold", so you hold Bitcoin because of its scarcity. You only gain anything if there is and increase in demand for it, otherwise at best it breaks even for you or loses value.
Eth offers a potential financial incentive for "locking" it up on their network. That is where it's store value will come from. Locking up your Eth will eventually be like owning "digital real estate". You sit back, collect rent and don't need too worry much about it losing value as by its very nature there will be demand for it..
Once the gold rush is over everyone will naturally want to migrate to property. If this stays a 2 horse race, then it's only a matter of time before Eth overtakes Bitcoin (admittedly it could still be a long ass time)