There are two problems with quoted text.
First problem is "if". In "if e.g. it turns out that the risk of mission failure...". The problem is - what if not? What if those "much tighter margins" and "much higher stress" nevertheless don't translate into substantially higher - and unpredictable - mission failure risks?
Then all those anticipated savings will just keep accumulating.
Second problem is with costs of cargo. There are reasons why a lot of cargo going to space is expensive. A lot of those reasons are related to the incentive to make cargo working on the first try. You can't go and fix things, saying goes, so you have to make your payload reliable - on the first and often only attempt. That's a good recipe for raising costs.
This is admittedly not the only reason. You can imagine unique enough telescope which would be as hard - relatively speaking - to build on Mauna Kea as it would be to launch to Sun-Earth L2. But even in those cases you could perhaps split the system, build pieces with some spares and assemble them - given that you could actually fly, repeatedly, there.
And for all others less exclusive payloads, that's even more true.
Now, the big reason why we usually don't consider that a viable option is because we used to think - over 60 years of Space Age - that Getting There Is Expensive.
But the whole point of many efforts - of both SpaceX and their likely-minded colleagues elsewhere - is to make getting there cheaper. First somewhat cheaper, then seriously cheaper, and then commodity-level cheaper. Then we won't need to create so unique payloads - because we'll have the benefits of available extra mass, for fuel, for construction margins, for extra systems, for docking, for service flights etc. We'll do building systems more and more as we do it with orbital station - instead of building unique Skylab we launch more specialized ISS modules, and much less of them are so critical to the whole project that we need to spend so much efforts to get them right the first try.
So this quote appears to me missing some important points.