No doubt some people get over-enthused over prospects who aren't quite knocking on the door yet.
I would just advise caution about believing that "established" major leaguers are rock-solid guys to count on either.
Take this year's Tigers, who are a big disappointment compared to last season. They signed 32-year old Framber Valdez. and so far he is laying an egg. "We all know" that players can start to fall off in their thirties, (And yet we have the Phillies who hit the jackpot by signing Zack Wheeler to a long contract starting at age 30. Grrr.) Zach McKinstry seemed to have a breakout season last year at age 30, but has slipped back to sub-mediocrity this year. But the Tigers must surely be dismayed at younger guys too; Spencer Torkelson and Kerry Carpenter are both at ages where you would expect to rely on last year's performance, and they've both contributed to the bad record for the team. Colt Keith came into his age-24 season with two years of service time and a good trajectory, and is scuffling this year. Pitcher Reese Olson is lost for the season due to shoulder surgery - we give certain players a pass for injuries as "just one of those things" but we are hard on prospects when they get injured on the grounds of "see, that's why you can't count on a prospect."
I don't know whether this year's Tigers are an especially good example or not (or if my quick and shallow analysis of them holds water). But uncertainty comes with the territory for any ballplayer. There are posters here who are just as adamant and certain regarding established veterans we should go and get, as the posters who dream on the next big prospect.
There being no sure things, we take our guesses as to chances of success and perhaps root accordingly.