I'll give you this. The one time a HLV has an advantage over multiple MLV's is when you have a payload that cannot be broken down into parts. Like a huge single mirror telescope. But you will pay out huge for that.
Given that Hubble and Webb cost about $7 to $9 billion (inflation adjusted, etc), I don't think NASA would have the money for SLS and a very large single mirror telescope payload, especially given the long, drawn out lead times on the Webb.
However, if a large space telescope could give them a justification for building the SLS (like space stations did for the Shuttle) I'm sure they'll start the design studies.
Frankly, the MLV concepts would aproach a large space telescope program by asking how hard it would be to form the mirror in space, because if you can do that you can build telescopes vastly larger than any conceivable ground-launch could deliver.
From the perspective of settling other planets, the next big threshold is actually seeing nearby alien planets, or at least getting spectral data from them.