> "just do the parts of it we actually use".
25 years here. You can absolutely do this. Most software is orders of magnitude more complex than it needs to be.
The junior programmer you are talking about who wanted to rewrite it in a weekend tends to come back with a working program, not empty handed.
I've seen this happen with both juniors and seniors. They do come back with a working solution /for the happy path/. Because the happy path is easy. It turns out that most of the complexity sits in the unhappy paths.
Yes, I didn't really doubt the developer could do it, the problems are:
1. That's not a great use of the developer's time, and
2. anything in-house increases our training and support costs
I agree. Just because you can buy some piece of software doesn't mean you should -- there is a lot of software that exists just to sell more consulting hours and will never fit the business. It's actually not hard at all to code and maintain much simpler alternatives.
Actually having to support multiple businesses with commercial software is hard. I've written a ton of custom software that far surpasses the capabilities of commercial offerings but if were to turn that into it's own commercial offering it would be large undertaking.