IMO, this is the value of Temporal. You certainly can write your own system—but wouldn't you rather focus on writing code that directly addresses the unique value that you or your company offers?
Build vs buy is hard. Generally you're correct, but then when your tool gets bought out by eg. Splunk (or whomever) and they raise the price 2-5x, you're kind of stuck.
Every 3rd party dependency is a complexity you must manage.
Every in house system is a complexity you must manage.
well theres several ways to answer that. first off characterizing TAM as "every business" is way too broad; up to you how to scope it down but lets say "data analysis for every business". There's not infinite spend for data analysis software.
and on the Temporal side, not B2C directly, but serving large B2C. We now can cite Snap as a public customer - every Snapchat story runs through us (and yeah, theres not infinite spend for orchestration either so make of that what you will). and we have bigger users than Snap.