It is my understanding Ethereum's best use is as structured smart contract (trust-less contracts). The cost for running such a contract is ETH. The value of ETH is it usefulness as a sort of escrow.
So if the cost of ETH becomes too high, ie. expensive, its usefulness diminishes. As the crypto space matures ETH and other "smart contract" tokens (LISK, NEO, ETC) become commodities and the most cost effective coins/block-chains will be used.
This can be compared to energy; oil, natural gas, solar, hydro, etc. In a mature crypto world there will always be market pressures keeping these type of coins priced as low as is economically feasible.
For this reason I agree that short term ETH has speculative value but long term it has a relatively low ceiling.
Bitcoin => gold
Litecoin => silver
Ethereum => oil
the specialized tokens => natural gas, diesel, coal, solar, hydro, geothermal, uranium etc...