And it's not unusual to call one of the LARS stations and be told - if you can get a word in on the R/T at all - "unable to give you service due to high traffic density". The same problem would apply to your TIS if you made transponders mandatory; you wouldn't be able to make anything out due to the number of targets.
This is nonsense I'm afraid. It's rare to be refused service with Norcal (SFO and a long way beyond), but it happens. And still the targets you need to worry about are very easy to see on the screen. And as always, mostly impossible to see with the much-vaunted "Mk I eyeball".
Just before the US eclipse event, we were flying north to see it. Oakland Center were clearly overwhelmed by the traffic - they later told me they were tracking over 100 GA flights, just in one sector. But still the ADS-B picture 6 miles out was nearly always empty.
There was one target that was close for a while. Once again we saw it on the screen LONG before we saw it visually, and hence were able to be sure to keep clear of it. Later we chatted to them at the destination airport.
Flying into Palo Alto at the weekend you can have a dozen or more targets in a 6 mile radius, but it is VERY easy to see which ones you need to be worried about.