I don't think it is quite as simple as that. Eflin had a really low strikeout rate as a sinkerball pitcher and that approach really does not work well these days. Guys who K 6/9 just don't survive. He does have good velocity though (not really great movement) on his fastball, so the higher 4-seamers were an attempt to give batters a different look and simultaneously raise his strikeout rate. This has worked to some degree.
One still needs to make good pitches though. And maybe Eflin needs to be a hybrid of a sinkerballer and a high fastball pitcher to be the best he can be. The lack of missing bats is really why he is a back end 4th or 5th starter, but he is also legit in that role. And a back end starter that can eat innings with relatively low pitch counts has value. It is almost why Velazquez needs to be a 3rd starter or better, because he does not eat enough innings in a back end starter role.
I do think it is looking likely that we will add a starter. I am not sure who should be replaced, but a competition between Pivetta and Eflin and VV to stay is not the worst thing. The big question is whether they can acquire a major piece without giving up one of their top prospects. The way he is playing now, everyone will be asking about Bohm. Might be one of the reasons they just drafted another infielder in Stott. Not saying they have a plan to trade Bohm, but if you want a top pitcher that is probably the price. Medina will not be the top piece in such a deal.