One advantage of System V is that it was available
I mentioned recently that even though I have a negative view of System V, it made its own contributions to Unix. There are a number of technical contributions, but one of the under-appreciated things it did was simply that System V was available.
The reason that Unix vendor after Unix vendor used System V is in large part because AT&T made it available for licensing. My understanding is that it was even licensed on relatively generous terms; it didn't cost particularly much money to get a source license with binary redistribution rights, and AT&T mostly let you do whatever you wanted with the result without many restrictions. The results of this is that from the mid to late 1980s onwards, Unix versions flourished everywhere, from the small to the large. We probably would not have the broad Unix ecology we do today if AT&T had tried to be more restrictive.
This is all the more noteworthy because AT&T itself was in the business of selling Unix machines for much of this time, and they weren't particularly successful machines either. AT&T undercut its own Unix server business by selling AT&T Unix licenses to direct competitors who then generally offered better products with it.
(Although I don't know for sure, I don't believe that AT&T required things like per-system royalties in its commercial Unix licenses.)
Now, I don't know how much money AT&T earned from its Unix licensing business. But either way, AT&T made an unusual decision to let its server hardware business suffer when it might well have been able to give it a hand, a decision that many companies have decided differently. The result of AT&T's decision here drastically helped Unix spread, especially in the basic server market that became the bread and butter of many Unix vendors.
So, regardless of what I feel about System V at a technical level, I have to respect it simply for being available, for being out there in the world and introducing a great many people to Unix.