There's some fun to be had in comparing players like this, but I don't put much stock in it. It's so hard to compare these players, nevermind the fact that I never saw most of the old guys play themselves.
I'd much rather compare Pierce against the players of his era, but maybe that's just me. They played in the same rule set, similar pace, against the same competition, etc. The NBA used to be much more talented when Bird played, though I'd say it's back to being that talented now and in the near future.
I'm not going to pretend to know if Pierce was better than Dave Cowens, I have no idea. What makes it extra silly is when we younger fans talk about players we never have seen, but like I said there is really no way to compare even if I had. It's just not really that possible.