Much better imho for a growing business to use their own in-house tools that they can mold how they want and that won't change UNLESS they want them to.
If you're spending 100K in integrations you think building an entire service, or suite of services is going to be LESS than that? Now take into consideration opportunity costs. This is insane.
I'll be generous though and believe that you're talking about using open source solutions. That's a more viable option, but you still have to do integrations so the cost is effectively the same, probably more because Open Source solutions are typically far less polished.
And even Open Source projects change dramatically over their life. Everything changes. That's technology. Better to get better at dealing with change than try and hold the world still.
Look at that discontinued list and look at the business services that have been discontinued, can you find any? Can you find more than 5? Look at G-Suite + GCP (Google's business offerings) and tell me how many there are. There are dozens. Many of which have been around for 10+ years (Email, Docs, Sheets, Analytics). So if I'm a business, making a business decision about a new business service from Google it's an incredibly safe bet that this service will be around for the long hall.
You can point to consumer services to try and discount that... but you'd be wrong to, and I think you know that.