In a ton of cases you'll be completely fine serving a few million monthly page views of your SAAS app on a single $20-40 a month server using Flask, Django or whatever Python web framework you prefer. Performance will be really good too. Just talking about most apps where it's mainly reading and writing data to a DB.
I write all my backend code in Go (used Python before Go existed) because I can deploy Go executable on the cheapest render.com server and it runs using fraction of the available resources (~50 MB out of 512 MB available and literally 0.01% CPU use).
If I used Python (or Ruby, node or even Java) I would likely have to go higher (and pay more) because those languages are not only slow, they are also memory hogs.
I'm as productive in Go as I used to be in Python, I see no reason not to use Go.
As a frame of reference, ASP.NET services in C# can exceed the throughput and performance of an equivalent Django app, but Django (and Python) may be the best choice for some people. What's not considered here is the experience of the developer and whether they can avail of the platform capabilities fast. Do they need to rewrite libraries? How much work is needed to be as proficient as you are with Go? The language and platform choice is rarely a differentiator here.
Over the long-term you can see some patterns. Python companies are able to scale up their teams more quickly and cheaply in some markets than Go ones. You choose to optimize hosting cost while they optimize other costs. They may need to avail of libs like Pandas and these are readily available. I don't know what the situation is with Go today, but similar libs weren't available originally and the Go community didn't have a big foothold in data science.