First, I agree 421a is ludicrous, which is why I don't see why you credit NYC for "extracting concessions like low and moderate-income housing". 421a and similar programs trade billions of dollars in tax revenue that could be spent on everyone for hundreds of thousands of dollars in rental subsidies handed out to a few lucky lottery winners. That is terrible public policy. A park or subway station improvement is a little closer call, but cash ought to be the gold standard.
Second, I'm not sure I care whether billionaires buy castle in the sky in midtown or townhouses in Greenwich Village. I wasn't going to be able to afford the townhouses anyway. It seems like the people most concerned about billionaires bidding up property are the centimillionaires being priced out. 100 units, even extremely large units, are a drop in the bucket for NYC. The knock-on effects are just not that large out in the places where the middle and lower classes live.
Finally, in terms of extracting more, I think the in-kind programs are both inefficient and a massive opportunity for corruption. I'd much rather see all the special deals -- parks, affordable housing, etc. eliminated and property taxes that reflect reality imposed, with perhaps a small pied-à-terre differential. The billions of dollars of missing tax revenue would be a lot more valuable to NYC than a few more housing lotteries or a small park, if for no other reason than you can subsidize rentals with cash but you can't fix potholes with subsidized rentals.