Search engines have two distinct parts, the engine itself - the algorithm which tries to guess at what you're tring to find - and a database of web pages. The database is built up, and updated, by automated spiders or crawlers.
A new search company can be formed by anyone who believes that he can come up with a better set of search algorithms. This company can start operating more or less immediately, without taking the time to build up a database, because several of the major databases are available by subscription. If the new algorithm is indeed superior to the old ones, the company may eventually supercede other search engines, even those using the exact same database. Alternately, should a larger and more complete database become available, the new company could use its new algorithm on that improved database. And of course, new website database-gathering companies can enter the field at any time. Cheap storage makes this possible, and it gets cheaper by the day.
The thing to note is that no company can prevent another company from either putting a new search algorithm into operation, or from forming a new database. Nuisance suits based on alleged algorithm patent infringements might sometimes work, but are likely to be minor factors in the long run.
Therefore, should Google go astray; say, by allowing paid placements to dominate rankings, or by censoring sites (guns, sex, politics, competitors) by gimmicking either the algorithms or the databases, it will soon find itself eclipsed by other search engines which don't do the same.
The only company currently in the search engine business which I'd suspect of gimmicking results (not so much now, but possibly in the future) for PC reasons would be MSN. The spiders which are most active on my web sites are Googlebot, MSNbot, Inktomi Slurp (the database used by Yahoo Search), Alexa, AskJeeves, and several which my usual stats packages can't identify. That's quite a few, and there may well be more in the future. So even if one or more start gimmicking searches (for our own good, of course!), who cares?