This is a key point to many things. Need to identify the difference between technically viable and economically viable.
It is the same argument I make for Starship. Technically, yeah it should work. The economics of of going to Mars, not so sure about that.
If they can get Starship reliable enough, it'll definitely be successful even if it never goes to mars.
But despite SpaceX's huge successes with Falcon, that doesn't prove they'll cross the finish line with Starship. Falcon and specifically the Merlin engine made just about every choice they could in favor of risk reduction rather than the best possible performance. It was a continuation of work that Tom Mueller did while at TRW.
With Starship they're in more uncharted territory, especially the re-entry concept, and the engine has been designed under the opposite philosophy of trying to push limits.