I can understand the contrast you projected. Having used both, I feel both have pros n cons.
When u have huge scales, paying for SaaS makes it too costly. Also the controls are less when you have to take it via third party. Some of the compliances force you to keep things in-house.
SaaS works best when you have low to moderate volumes. Plus you don’t have engineering capacity where any functional team members can tag the events and reflect on dashboards.
If you are loving mixpanel, try exploring Pendo. Both are gold standard tools.