You are right in a way SDSU is no comparable replacement to USC/UCLA but there is a little more to it. It is not exactly the same.
Sam Houston competes with Houston for the same market. SDSU does not compete with USD, or UCSD. Secondly, almost all of SDSU's football and basketball games are televised nationally already. Lastly, The Houston Cougars have always struggled for brand relevance within their home TV market, especially against Texas and Texas A&M.
This Google Trends analysis of football teams from 2004-present shows that Houston has had roughly half the relevance of the Longhorns and the Aggies within the Houston media market, and even trail LSU by a good margin.
S
an Diego St is at least the top brand in their home market which, in 2022, was the 28th largest with 1.11M TV homes (though San Diego lost 25,290, or -2.3%, of their TV homes from 2021 to 2022). The Aztecs claim nearly 38% relative search interest in San Diego when compared to USC, UCLA, Notre Dame and BYU.
There are 1.1 million households being added to a market in a region where the Pac 12 really needs it. (it is a mid-size market) but it is a region that the Pac 12 really needs.
If you look at the Big 10, Maryland and Rutgers did not "own" their massive tv markets many think the media financial calculus has shifted away from total TVs/markets and more towards stronger brands and rivalries)
So, yes you are right SDSU is no comparable replacement, but they are the best fit for the PAC-12 and a no brainer addition. Yet, if the Media deal is not a great upgrade for them, they should not make the move as they still have a lot to offer the right partner, and that may be the MWC for the foreseeable future.