The keyword is "building". Yes costs may have dropped 90% just to build software. But there are 1000 other things that comes after it to run a successful software for months let alone years.
- Maintenance, Security
- Upgrades and patches
- Hosting and ability to maintain uptime with traffic
- Support and dealing with customer complexities
- New requirements/features
- Most importantly, ability to blame someone else (at least for management). Politics plays a part. If you build a tool in-house and it fails, you are on the chopping block. If you buy, you at least can say "Hey everyone else bought it too and I shouldn't be fired for that".
Customers pay for all of the above when they buy a SAAS subscription. AI may come for most of the above at some point but not yet. I say give it 3-5 years to see how it all pans out.