There is a bit of a Ship of Theseus question going on here; but if you built your modular laptop 10 years ago, then you have a 10 year old laptop.
A modular design offers more opportunities to maintain and upgrade your laptop (potentially with modules more advanced then what was available at the time you built it), but at the end of the day, you are maintaining and upgrading a 10 year old laptop, not constructing a new one.
Of course, even the ability to meet this promise depends on the continued availability of parts in the old form factor.
If a sealed laptop lasts 4-5 years then a modular one should last 8-10 years to be worth bothering. Future-proofing generally isn't compatible with rapidly advancing tech but maybe the slowdown of Moore's Law will make modularity more worthwhile.
My HP ZBook 15 from 2014 is still good. I upgraded it to 32 GB RAM, 2 TB SSD starting from 8 GB and 750 GB HDD. I replaced several keyboards (usually because I worn out the most used keys) and a defective screen when it was under the original guarantee. I hope it won't break. I can see me using it for many years, no need to buy something else. If only HP would offer an option without the number pad.
Why does it need to last twice as long? If a sealed laptop lasts 4-5 years then a modular one only needs to last a bit more than 4-5 years to be worth bothering?
A modular design offers more opportunities to maintain and upgrade your laptop (potentially with modules more advanced then what was available at the time you built it), but at the end of the day, you are maintaining and upgrading a 10 year old laptop, not constructing a new one.
Of course, even the ability to meet this promise depends on the continued availability of parts in the old form factor.