If anyone is curious, the odds of failing to finding a 4-of in a 60 card deck with 6 mulligans (as 7 mulligans serves no point) is (1-0.40)^6 = 4.7%
You get seven shots at finding the Bazaar (keeping 7 cards, keeping 6, …, keeping 1), so the probability of failing to find it under the new rule is (1-.4)⁷ = 2.8%. I don’t know exactly how Serum Powders will work under the new rules, but if you just exile seven, I don’t think main deck Dredge hate will be good enough.
I simulated 1000000 games for each option (old/new rule, with/without Serum Powder). These are the percentages of getting T1 Bazaar:
Old rule without Powders: 86.6%
Old rule with Powders: 94.1%
New rule without Powders: 97.2%
New rule with Powders (exiling 7): 99.6%