I get a new Mac at work every four years so I'm not sure. This is my first personal iMac. But I can answer for my friend, who upgraded from a 2009 iMac, so about 10 years. I doubt I will ever go that long for daily use, but I plan on keeping this 2019 iMac in the family. I think it will be a great family computer for my kids someday. I do plan on teaching them how to use and appreciate Macs. I gave my daughter her first lesson the other day on how to use a mouse-based UI to set typography and she is 5 and she picked it up fairly quickly. They currently use iPads for about 30-45 minutes per day. The iPad is pretty locked down. I'm hesitant to get them Macs until they are in high school if Macs are even a thing by then (God forbid) because I don't think I can lock them down as tight. The internet is really freaking scary for kids nowadays. But I would let them use the iMac in a family space for doing schoolwork such as research papers that require lots of multitasking with windows side by side. Maybe by then the iPad will be better suited for tasks you can do on a Mac but who knows. I think a big display is still superior.
I can understand your reluctance with giving your kids a more privileged computing environment. As you say, there are scary things on the internet.
But I would urge you to, when they are a bit older than it seems they are now, allow them a fully privileged computing environment.
Before I go on with my opinion, I'll just get it out of the way, that I am merely some random 23 year old who has 0 experience with parenting; I've done some reading, but it's not even close to anything in my immediate future or anything, so what I'm saying now is not as much about parenting, as it is about my own experience with growing up in basically full computing freedom.
Computers are all around us all the time, and whether we call it as such or not that includes the iPad, iPhone and any other device with a processor in it.
The biggest difference between a 'conventional' computer and these appliances it how easy it is to explore the workings of a 'computer'. And really understanding the technology that surrounds us is so crucial these days, just because there's so much of it.
My ex-girlfriend didn't have the admin password for her laptop for a while, because her dad had set it up for her; It wasn't that she couldn't have it, he just forgot to give it to her. - It limited the computer's use dramatically, to the point that she sometimes couldn't use it in class. We met in high school, and until she got the password, which took a while, she couldn't print at the school, since that required the admin password to install drivers. She couldn't install the program we used to draw molecules for chemistry. There were even updates she couldn't install, and at one point, after some scheduled Windows update or something that had apparently been authorised, she couldn't even log in for the rest of the day.
Now that was with Windows, and I frankly think a more granular lock down couldn've been possible with macOS allowing her to work more effeciently even without having the key, but these are just a few examples and not even my main point anyway.
What I'd consider the biggest issue with locking down your kids' computing privileges, is limited access to the Unix shell. I know that not everybody are in training for a computer science career, and that's fine, but I would hate for someone who wants to explore to be blocked from that learning experience.
Furthermore, it's a trust exercise - Your kids will become adults one day, and the best part of granting your kids privileges, is that you show that you trust they'll use those privileges wisely; This of course depends on the kids having an appropriate age, and the potential danger from abuse of those privileges, but if none are ever granted whatsoever, when they grow up and become adults, they might not be able to properly adjust to a life of full privileges, and abuse the power they have over their own life in self-detrimental ways.
I am therefore a big proponent of giving as many privileges as appropriate - what is appropriate is very case-dependant, but I think in the majority of cases, full computing rights is almost a must. Especially when a time comes when your kid buys a computer out of pocket - As long as you buy it and let them use it; well, conditions may apply, sure; Though even then I'd argue for administrative rights in a lot of cases.
This was really off-topic, but since it relates to a post in this thread, I still figured I'd post it here.
To stay somewhat on topic, I guess I'll add that in addition to my 2014 iMac, my 2000 iMac, and my 2003 iMac, I also have a 2014 MacBook Pro running perfectly, an iPad Air that's slow as pancakes, but works and my 2011 MacBook Pro died from GPU failure, but was covered by a replacement program since it was a known issue, I had already purchased my 2014 to replace it when the repair program was announced, but I did get the logic board replaced for free, significantly increasing resale value.