Agreed in general, but who is the third player who is starter quality for a playoff team? Are you talking about IT? Agree he is obviously starting quality when he is back up to last year's form but the jury is out on when / if that happens.
We do have some role players who could be fifth starters on just the right kind of good team -- TT on a team which was an offensive powerhouse and didn't need him on that end, and didn't need a rim protector, Green on a team that could absorb his occasional fuckups, etc. But none of them would be plus players overall as starters on a playoff team.
Once again: our ROSTER is defective. That is the fault of the front office and the organization, which has continually screwed up and made mediocre decision after mediocre decision over a period of years. From a short run perspective they did make some good decisions at times (as evidenced by a championship), but even those decisions did not pay off over the long run. Overall they have put us into a hole that it will be difficult to get out of. Blaming Lue and the like is missing the point.
Our ace in the hole is that we have Lebron and he needs less supplementary talent than other players do to make his team relevant. But in the era of the Warriors we do clearly need a roster upgrade. In terms of your proposed solution of trading depth for quality, unfortunately it is very difficult to package the kind of bench players we have in a major trade. The players on longer contracts are pretty clearly overpaid, and the vet min guys are people with little value (DRose, Calderon) or people we wouldn't want to let go (DWade, Green). Our trade sweeteners are Cedi and a first rounder, and we can't let those go without getting the perfect return.