You start with an $800 laptop that is now worth $200 and junk it over a $150 screen or board replacement.
Okay... You still have to replace that laptop, it's unusable. You just spent $800 to fix a $150 problem.
Yes this is exactly what you do because you don't spend $150 to try to salvage a $200 machine. Especially because you don't know that the $150 you spent will actually fix it permanently.
It doesn't matter if the laptop is only worth $200 now, it needs to be fixed or replaced and you are not taking into account everything involved in replacing it.
You still need to move your data, setup your printer/scanner network, familiarize yourself with a possibly new OS, reinstall your software, make your preference changes... Do you know how to recover all your data or was it backed up recently, can you restore it? Do you have the equipment? For my customers in your scenario, they just bought an $800 laptop to fix a $150 problem, and they are still going to be calling me out and paying $120 to get their data moved over and setup everything. So it's not $800 to fix the problem, it's $920 to fix a $150 problem, and that "fix" still means taking time out to go buy the laptop, scheduling an appointment and waiting until I can get out there to do the job which itself will take a couple hours. Then they have familiarize themselves with all the changes because it's almost never quite the same as it was no matter how hard I try.
Of course time to move data needs to be considered. If you have your data in a way where this is really problematic then yes, it might not be worth it to do this. It could even be it costs $500 to fix it but then that doesn't really matter right? This is a different problem though. That isn't about extending the life of an old Laptop..it is actually the inconvenience or dependency you put on this machine.
So let's go the other way. Let's say you do buy the $200 copy of what you had because the screen broke.
How much time did you spend finding/buying and waiting on delivery? You still lack all of your data and you have an unknown variable, the laptop itself? Is that a dead pixel, is that hinge a bit wonky, how is the battery? Is that fan failing? You just opened yourself up to a whole new set of problems as opposed to just fixing what you had and you knew worked. Yes, you have spare parts, but what if the part you need next time is the same part that failed last time. You haven't accomplished anything.
Or eliminated those problems that might've popped up with the older laptop. This argument goes both ways. I'm not a fan of buying used computer equipment for this very reason but it isn't any worse than getting it fixed.
Lets take a realistic example.
Work PC has gone bad. You can either get it fixed.
Get it replaced with the same laptop but used as your IT person has spares. This means you can swap the drive.
I'd go for door number 2.
A motherboard requires no software changes or anything to replace, it's about as difficult as replacing a screen. A T440 motherboard swap is about $150 and a T460 motherboard swap would be about $250. I can usually swap a Thinkpad motherboard in 45 minutes or so if I'm in a hurry and the user is back up and running immediately. A home system will cost more because there are fewer parts (if any) available, even fewer certified good ones, and it takes longer to do the job due to the construction.
Not sure what you mean by home system but I assume you mean consumer rather than business parts...but businesses aren't hiring people to stay on their IT staff to fix hardware issues. It isn't an efficient use of time or money nor does it even make sense from a $$$ perspective. You replace it and move on.
By comparison, a drive or system replacement can take hours.
It's not the drive cost or even the labor to install it, it's what comes after. You have data recovery if possible, reloading Windows, re-installing software, you have to re-learn and re install things you used to have like browser extensions etc. It can take weeks for a customer to get back to where everything works like it used to. Replacing a drive is one of the biggest pains in the neck I (and customers) have to deal with. There is more labor involved in replacing a drive than it is to replace the system and it's just as hard on the customer to get used to after. This is why I say drives are what usually kill old Lenovos. It's a combination of cost, age and hassle.
If the data is backed up this isn't a big deal. You image the drive and you're back up and running. Yes whether its the drive or laptop, there is work involved.
The question is..
3 years into your laptop..it has an issue.
You can spend 150-200 to try to fix it...or spend 600-700 to replace it.
200 to fix may mean your laptop is fixed and everything is good...maybe..and you don't have to spend time dealing with it. At the same time your PC is still slower than it could be and you have older technology.
Or 700 to replace it.
You will need to reinstall your stuff..and spend a few hours doing that but you have a new PC under warranty..you have new technology, you have a faster machine and a cleaner machine...
Each person is going to place their own value on that....but businesses are not stalling out replacing PCs for 5-7 years because that's just inefficient in many ways. Consumers will need to judge how long they want to try to stall that out but in general it just isn't try to stretch out PCs...your chances of problems jump dramatically...you have a slower and less power efficient machine as well..
I have older Lenovos...they're just not worth fixing or doing anything than running very basic stuff..so much so I've just put linux on them instead...if they broke I'd shrug and move on..
