I don't see what having a tower has to do with being able to maintain an altitude.
Draw a horizontal line between HAF and SQL, and that whole area about 5 nm SSE of SFO can be a real cluster. SFO is not a particularly friendly Class B tower when it comes to VFR bravo transitions over the airport (which we generically tend to call it the "Bay Tour"), and if you're heading northbound to that spot on the map and need to get by SFO, there are basically
three four ways you can go (you can also climb above their airspace and just try to deal with norcal).
1.) Straight through with the "101 transition": "Cessna 12345 cleared into the bravo airspace, remain west of the 101 freeway, maintain VFR at or above 1500." Can put you in the way of heavy departures off 28L/R, and SFO routinely denies these due to workload.
2.) West end of the peninsula with the "280 transition." Also requires a bravo clearance, and gets you even more in the way of any potential departures from 28L/R so it's a crap shoot.
3.) From SQL, turn NE to the midspan of the San Mateo bridge (a little scary without GPS since the thing is so long and the 'kink' in the bridge isn't actually at the middle so it's hard to tell where the bravo starts), then contact OAK tower and overfly the runway 29 numbers and midfield before proceeding on course.
4.) Fly offshore between HAF and Lake Merced at or below 1500 and don't talk to anyone. That entire coastline is high cliffs with no place to land, so in the event of an engine failure you'll be ditching into some really cold and treacherous water. Not my first choice.
In cases 1 to 3, you'll need to be in contact with some busy ATC facilities (norcal approach, SFO tower or OAK tower). SQL tower can be a huge help because they'll get on the interphone for you, coordinate a clearance and get you a beacon code. In other words they act as kind of a filter and help space out the traffic on 3 out of those 4 routes around or through the brick wall that is SFO. They have LOAs between the two facilities and won't send a bunch of people into SFO's airspace all at once. I am of the opinion that if SFO tower already knows who I am, where I am, what I want and that I already have a beacon code, they will be much more inclined to let me through because their workload on the frequency is lower.
I've been inside SQL tower on a tour and met some of the controllers there, so a lot of this impression is based on that. I don't think the world will come to an end if they close it, but it will be a very unfriendly place to fly into given the amount of landing/departing traffic and transitioning overflights all converging in the same area. And while most of us can maintain our SA and stay out of the bravo, with all the shelves around there it'll be a lot tighter quarters than what normal uncontrolled airport pilots (some of whom can be "special", speaking from experience at my home field) may be used to dealing with... and there will be no controller there to yell at them should they start veering towards the SFO final...