1) It would create a bad product, according to Google's own prior statements on the matter.

2) It would encourage government authorities to look even closer at potential abuses toward consumers (ie what kind of consumer harm is occurring, if any, when a monopoly search provider prevents attempts by consumers to use ad blockers; that would have to be resolved legally). European authorities would likely pounce on such a move immediately, looking for any opening by mistake to go after the company, given how they've handled Google thus far.