what if the expensive SAAS offering is just as vibe coded and poor quality as what a junior offers?
A typical SaaS customer will use many pieces of software (we mostly call them SaaS now) across its various functions: HR, accounting, CRM, etc. Each one of those will have access to the same pool of senior devs and AI tools, but they will pour more resources into each area and theoretically deliver better software.
The bigger issue here is the economics of the C-suite have not changed here. Assume a 100 CPG company uses 10-20 SaaS apps. Salesforce might be $100k/year or whatever. 1Password is $10k. Asana $10k. etc. They add up, but on the other hand it is not productive to task a $150k employee with rebuilding a $10k tool. And even with AI, it would take a lot of effort to make something that will satisfy a team accustomed to any modern SaaS tool like Salesforce or Atlassian. (Engineers will not even move off Github, and it's literally built on free software.)
That's before I get to sensitive areas. Do you want to use a vibe-coded accounting system? Inventory system? Payroll? You can lose money, employees, and customer perception very rapidly due to some bugs. Who wants to be responsible for all their employee passwords are compromised because they wanted to save $800/mo?
Then, the gains from cutting SaaS are capped. You can only cut your SaaS spend to zero. On the other hand, if you have those engineers you can point them at niche problems in your business niche (which you know better than anyone) and create conditions for your business to grow faster. The returns from this are uncapped.
TL;DR; it's generally not a great idea to build in-house unless your requirements are essentially bespoke.