Actually the math is worse.

Every new line increases dramatically the complexity of the software which requires more cost and most maintenance. If you stop at a single tool this is still manageable.

Imagine now that you do the same for 10 other products - all critical systems. You will end up running the tools to save on imaginary money. Even then, software vendors can simply undercut and offer the software cheaper because they use agents too.

So building everything in-house is not the right way to go unless you have no other option - which was always the case pre coding assistants.

If you ask engineers of course they will say yes. It is yet another nice toy project and interesting challenge. But decision makers need to learn to say no - more often they used to.