Consider a rocket using nuclear fuel, where the energy density is far higher. Let's stipulate that in an SSTO configuration, it's got a payload capacity like the Falcon Heavy. So, at that point, you don't need the boosters...but it seems like with the boosters, your payload capacity is higher still.
Is there a tipping point where boosters no longer make sense or are you always better off with boosters using the same high-density fuel as the main vehicle?