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.
There's an engineering trap/fallacy I like to call "how hard could it be". How hard could it be to build a [whatever] clone? If you find yourself thinking that, stop what you're doing, because the answer is almost always "at least an order of magnitude harder than you think."
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.
1. That's not a great use of the developer's time, and
2. anything in-house increases our training and support costs
2. If the in-house software doesn't decrease training time or support costs then there is something wrong there.
There's no point in saving $20K on an SaaS app if you use $100K in developer time and miss out on $1M of potential revenue. We get paid the big bucks because we can make companies a lot of money.
2. Haaaa no, that's 100% not how that works. If you buy a SaaS product, the company made that product. They have documentation. They have training. You can hire people who have worked on that system before. If it goes down, they get paged.
If you write the tool, all of that is on you to do. If it goes down, you have to fix it. If it screwed up data, you have to fix it. Any time anyone has any questions? Guess what, you're the one they'll ask. All of that costs the company money, because you don't work for free. When you quit, the app is now useless and can't be fixed unless you did a lot of work beforehand.
It's best to think of DIY apps like those really really sticky noxious tarpits. It might look safe or easy to get into, but good luck getting out of them. You might end up at the bottom with the bones of everyone else who thought that DIYing it was a good idea.
