That happened a decade ago. Users dropped from Facebook like flies and moved to Instagram. Mark Zuckerberg's response was to buy Instagram. The Obama DOJ waved through what was obviously a blatantly illegal merger.
Likewise, Google's only ever made two successful products: Search and e-mail. Everything else was an acquisition. In fact, Google controlled so much of the M&A market that YCombinator (the company that runs this forum) complained in an amicus brief that they were basically being turned into Google's farm league.
So long as companies can be bought and sold to larger competitors, no tech company will ever become irrelevant. They'll just acquire and rebrand. The only way to stop this is with the appropriate application of legal force.