(1) It's a lot of work to migrate away from a platform like that, particularly if you've customized your org heavily.
(2) Early on Salesforce.com had some good patents, particularly one for a self-optimizing database (would fix a lot of what is wrong with RDF databases...) I'm pretty sure that patent is expired now.
Speaking of patents, I thought Lotus Notes never got a close competitor in the day because IBM held so many patents for it. Those patents have to be expired, but for all the interest in document databases and "low code" now I haven't seen anything try to be "the Lotus Notes of 202x".
Overall, Salesforce's technology looked long in the tooth in most ways when it was 10 years old, it's not sure if their way of doing multitenant was the right way, etc. It's a highly reliable system that performs well, but also dependent on Oracle and gold plated (expensive to run.)
Salesforce was able to pass on the high costs of their system to their customers (it costs a small fraction of the salesperson that it controls) but it kept them out of applications that were more cost competitive.