Its probably easier to leave the mulligan rule as is for eternal formats than have to deal with multiple rounds of bannings/restrictions before they got this right. The odds of them deciding to restrict bazaar and workshop are lower than the failure rate of finding a bazaar w/ serum powders. I mean do you keep bazaar and restrict what?, grave troll, narcomoeba, and serum powder? Maybe hollow one? Is leyline of anticipation.dec automatically tier 1? Would we be locked into additional shops restrictions? How unfun would game 2/3's be on the play vs blue.dec when they can aggressively mull to FOW? Do all these things balance out in the wash? Far too many unknowns for my liking when considering nearly unlimited card pools.
Also (chuckles to self) the idea that MODO would program the new mulligan + Serum Powder interaction correctly out of the gate is low probability as well.