With this many [cancer] drugs in development, I think the success rate will probably decrease.
If you define success from a commercial standpoint, your assertion is guaranteed. There are only so many reimbursement dollars available in the aggregate for all cancer drugs, so it’s mathematically impossible for more than a tiny percentage of these drug candidates to be commercially successful.
There are exceptions, of course; however, all told, cancer-drug candidates are a sucker bet, IMHO.