I wouldn’t say people have adapted to the need for private Modules because people have always tried ways of keeping ownership of their code or unique ideas whilst allowing others to still use it on Roblox.
The earliest way was to compile the script into Lua byte-code and call loadstring on it (This was easy to reverse but it at least helped). The only people that I’d say have adapted to this paradigm specifically are those who have relied on selling private code as their primary source of income.
The main reasons why people want to keep their code secure are clearly explained in @IdiomicLanguage’s last reply (here) and are not issues that have only appeared as a result of private modules being possible.
In an ideal world where people don’t not steal other peoples work and benefit from it themselves and Roblox protect our IP from such acts, I’d agree that all public code should be open source because there’d be nothing for the original creator to lose.