The observation that the dark matter density is insufficient for flatness is essentially the same observation as the need for dark energy. It just has to do with the dynamical behavior of the expansion, because cosmological dark matter and dark energy really don't show up anywhere else except in the total scale parameter of the universe. If the observations we use to interpret the need for dark energy are being interpreted incorrectly, then the inferred dark matter density would likely be higher. I don't really know, but I suspect it is easier to exchange dark energy for dark matter than it is to just be without flatness-- the flatness might be a more robust result observationally.

Having said that, I might note that flatness is not a requirement of inflation, it is just the main reason one might invoke inflation. In other words, inflation is a useful way to explain the flatness, but it would be possible to have less inflation and a nonflat universe today. It just wouldn't be very helpful-- inflation is invoked to avoid fine tuning problems, and if the inflation isn't enough to generate apparent flatness yet the universe is still in the general ballpark of flatness, that's a fine tuning problem of its own. So what I mean is, those who favor inflation generally do so because they think any universe that is remotely close to flat is too finely tuned to not have inflation, but then the inflation itself had better not be finely tuned, so it must make a flat universe a very likely result for a wide range of possibilities. That sounds like what you are saying also, but I don't think there's a way to infer the dark matter contribution separately from the dark energy contribution to that flatness, they are distinguished entirely by their different effects on the dynamics of the universe's scale size (sometimes called their different "equation of state"-- how their gravity responds to expansion by virtue of their different pressures).