I've had similar thoughts about this development. (Not about myself being able to rewrite DOS without much effort - but I don't doubt that you could!)
The sophistication of some individual projects today rivals major industry efforts back in the early 80s.
I think it boils down to mostly these things:
a) back then the number of software developers was perhaps 100x-500x smaller than today - so there was simply a much smaller pool of smart people. (But also: Today we have a much, much larger pool of kinda meh people.)
b) there was no widespread internet access, so developers learned new concepts very slowly, often re-inventing them on their own. also, smart people often gut stuck in places where they had no chance of excelling.
c) better tools now: much better screens, windowing systems, mice, better editors (edit: not necessarily IDEs), source code version control tools, SSDs instead of floppies, VMs, trivial networking, pdf readers, archive.org/etc, web browsers/services/sites, etc etc.
The sophistication of some individual projects today rivals major industry efforts back in the early 80s.
I think it boils down to mostly these things:
a) back then the number of software developers was perhaps 100x-500x smaller than today - so there was simply a much smaller pool of smart people. (But also: Today we have a much, much larger pool of kinda meh people.)
b) there was no widespread internet access, so developers learned new concepts very slowly, often re-inventing them on their own. also, smart people often gut stuck in places where they had no chance of excelling.
c) better tools now: much better screens, windowing systems, mice, better editors (edit: not necessarily IDEs), source code version control tools, SSDs instead of floppies, VMs, trivial networking, pdf readers, archive.org/etc, web browsers/services/sites, etc etc.