Basically, nothing you've replaced so far is out of the ordinary for a well-used bike, other than perhaps the seat. I've worn out two front rings, two clusters, easily six chains (which I now replace every thousand miles, just on general principle), and at least a dozen spokes (not counting those I've replaced when I relaced "tired" wheels). Replaced entire wheel sets a couple of times. Also, two bottom brackets, three sets of pedals, and a couple of wheel bearings. Never replaced a freewheel, but they do fail on "good" bikes. This would be in maybe 10,000 miles of riding on three bikes. Someone heavier and/or more muscular (I had polio as a child) would likely wear things out about twice as fast as I do, and off-roading in a bike not designed for it will obviously take an early toll on the bike (as will riding a bike clearly meant for a smaller, lighter person).
I have heard of forks failing (rarely), and, unfortunately, there's no reliable way to know if one is about to fail. Some forks are very well built, others not so well, some are built such that cracks will be obvious, others have designs that will conceal cracks. But looking at the picture of your bike it looks like a reasonably good fork design that would not conceal cracks and not be prone to premature failure.
I've never heard of a catastrophic handlebar or stem failure, and I would judge them quite unlikely.
But there does come a time when it's best to retire an old bike and get a new (and hopefully better) one. I wouldn't discourage you from doing this, but it doesn't seem imperative in your case.