Legends SD top teams (NPL last season, E64 this season) are not very good. The majority of the ballers in East County/East San Diego (where Legends SD is located) already move to other clubs. Sharks will not be picking up any significant strength in the roster from the existing Legends SD teams. Also proximity isn't close 15-20 miles between the two.
Last season the Legends Santa Clarita Valley (SCV) branch had the Legends E64 teams. This season SCV took on the new RL SoCal leagues, and gave up E64. Legends SD picked up the E64 league, but their teams are not performing anywhere near the SCV teams. With E64 losing South Valley SURFS top teams, and SCV's teams, I expect the quality of the league to drop quickly. Certainly these clubs would be better off playing NPL instead of traveling all over SoCal to play teams that are the same level as NPL.
Sharks becoming Legends is a pure business decision. Who is benefiting from the deal? In a merger, it won't be both. I don't think Legends needs the Sharks, so the writting is on the wall. Most likely it is Legends pushing back on SURFS incursion into the IE. Not much will change the first year, but you can bet there will be changes next year.
What you (and I) believe to be true and what the Sharks leadership talked themselves into when they made this deal, are likely entirely different things.