Criticism by referee I got a report on one of my papers recently, in which the referee claimed my results were "natural". This was one of their primary reasons for rejecting my paper. I agree the proofs are not exceptionally difficult, but on the other hand I feel my results significantly advance our understanding of certain mathematical objects, and may be crucial in answering some open questions in the field. Is the rejection simply a consequence of my not adequately "selling" my results, or is insufficient novelty a good enough reason for rejecting a paper?  In time, I feel that I will be able to use the results to answer these big questions, but of course there is no guarantee of this. Should I simply aim for a lesser journal now, or is the referee's comment likely to be echoed by other reviewers, in which case I should "hold my fire" until (if?) I draft a more substantial paper?
Insufficient novelty is definitely good enough reason to reject a paper - otherwise for example I can just repackage the proof of Pythagoras's theorem from the nearest mathematics textbook I can get my hands on, and publish that (I can publish that multiple times too).

To publish your results you need to convince the editor that the result actually matters. You write that they "significantly advance our understanding of certain mathematical objects". That's the key point you want to elaborate on. How exactly do they advance our understanding? Do they show that certain approaches to the open questions in the field are more likely / less likely to be successful? Do they lead to any partial results?

It's impossible to tell how other (not necessarily lesser) journals will react, since peer review is an inherently stochastic process, but if I'm the author I would look carefully at elaborating on the above questions before submitting it to another journal.