Google says deleting old pages to bamboozle Search is "not a thing!" as CNET erases its history. The tech news site removed thousands of old articles in a recent bid to appease Google and improve its performance in search results over the past few months.
This is because of Google's monopoly on search. If there were more search engines, then sites would just focus on making high quality articles instead of trying to play with the monopolists policies
If there were multiple search engines (there are) then SEO would focus on whichever were most popular
I've literally done work where SEO included making sure Bing was also optimized for, and I know others who have also done so
More engines would make SEO harder and longer as we assume each engine would search differently, but SEO will creep into any engine that gets popular for the obvious reason of people wanting their content seen
I don't think they're necessarily so much the number of search engines that currently exist (there's already currently several) but rather that not enough people use the alternatives that Google had the monopoly. (Also helped by Google actively railroading users into its products and suppressing the competition)