So far as I know Big Agnes, MSR, and the other "big company" makers use the same 30D silnylon fabric in most of their "ultralight" tents that the cottage makers like Tarptent and SMD use. The main difference is that most of the "big company" tents are dome or dome-like designs that include extra weight of tent poles.
The dome-like designs with arched poles are great for someone who wants a free-standing option, but I don't see how they increase durability. In fact, having those arched poles to tension the fabric seems to add another point of failure, since these lightweight tensioned poles are more likely to break than a heavier-duty trekking pole in straight-on configuration in cottage-company designs. So aren't those dome-like designs less durable (when they use same 30D silnylon as cottage tents)?
I do see how tents like Hilleberg can be more durable; it's because they use heavier and more heavy-duty materials than ultralight tents. The fabric Hilleberg tents use is heavier than 30D silnylon used in ultralight tents (and I expect their poles are also heavier and stronger than those used in ultralight arched-pole designs).
EDIT: I should add that I don't think durability is an issue with 30D silnylon. It's proven itself over the years, many people have used ultralight 30D tents for hundreds of nights and thousands of miles, which goes far beyond what I expect to use them for. I don't even think you need to baby it to get that; just use common-sense care. Also, even the heavier weight materials are pretty delicate in some situations, e.g., resistance to fire or to sharp-edged objects.
