"Impossible" is a loaded word.
Apple is so big today that they could in theory make a plausible bid for any almost company in the S&P 500.
The fact is they wouldn't want to because quite frankly there's a reason why they're making more money than anyone else, why spend a ton of your assets on a corporate entity that quite frankly isn't anywhere near as profitable as you are?
People have been saying Apple is in decline for like 5 years now, yet they just posted their most profitable quarter in history and I believe the 2nd most profitable quarter in financial history period (Fannie Mae is no.1 but that's inflated by a shit ton of government cash).
Apple stock peaked around May of 2015, and it is now at about the same level as July of 2014. Even though iPhone sales are great, sales of the Apple Watch and iPads are below expectations. Apple is also predicting that revenue from the next quarter would decrease when compared to last year, which would be the first year-over-year revenue decline for Apple in the last thirteen years. I don't think anyone is saying that they're going out of business anytime soon, but it's quite possible that they have peaked.
Mind you, there are analysts saying that they may double in the next few years, so who knows...