I agree. Hiring devs and interfacing is the better solution.
Now, the dev team doesn't have to be in house, it could also be an external team. As long as they develop against and API, then that's all that matters. If it's easier to hire an external team to develop than making the longer term commitment for new employees, then do that.
What I haven't seen anyone mention is the cost of upgrading. This is the biggest factor by far. It can cost you more in upgrading what you've customised than the original cost to create it.
I've seen many companies get their first Oracle ERP system, and go bananas customising everything. We it comes to upgrading, they get the estimate for the cost to upgrade their custom code, and they get a shock. I know of places that have cancelled their upgrade projects since the cost to upgrade with customisations was too much.
What happens in these cases is that for the upgrade, instead of bringing everything along, they reimplement the system, and ditch most of the customisations, only including the ones are are absolutely needed.
If instead of making changes to the Oracle ERP, you just feed it the data from interfaces, you have now de-coupled the need to change your custom code from the ERP upgrade - at most you will need to make minor changes to the interface data if Oracle change their API.
This is really the best way to go. I've been consulting/supporting oracle ERP systems for over 20 years. If you want to contact me privately, I'd happy help you further. Now, even though I do ERP consulting for a living, unless you're in Europe, my company won't be getting any business from you, so you can trust that my advice will be independent.