You're making it sound like USV is struggling or becoming extinct, but my impression is that they're actually doing incredibly well.
First, they're considered one of the best Series A firms in terms of financial returns. See: http://fortune.com/2011/06/06/venture-capital-returns-the-be...
Second, they have an amazing reputation among founders. Check out the following survey, where USV was ranked 2nd out of 64 Series A firms in terms of who people wanted in their Series A round: https://www.cbinsights.com/research-venture-capital-series-a...
Finally, I'm pretty sure I've read this in several places, but USV is not "stuck" in the middle. I'm positive that if they went to the market and tried to raise $500m, the market would oblige happily. It's more that they've found their fund size sweet spot -- their product/market fit, if you will -- and have little desire to raise a lot more money and become a larger firm. A lot of funds do this: they raise larger and larger funds for a while, then stabilize once they find a fund size that works well for them.