Letting advertising in. Advertisers always seek to get favorable placement, treatment, more of everything. The old newspaper thing about editorial independence was always aspirational, if not an abject lie.
Google let ads corrupt search.
Google let ads corrupt search.
Strictly from a business standpoint, letting advertising in was Google's best decision. I don't see how platforms like Gmail or YouTube would exist today without it.
I am not so sure ads were the initial business plan. I think the initial plan was to take care of search for portals like Yahoo! and eventually sell the company to them. Larry and Sergey at one point asked Yahoo! for $ 1 B, were told “no”… and the rest is history.