Do you need supplements if a tabletop game is a competent product, meaning that it is a turnkey product that is complete and comprehensive given its stated aims?
No, you don't.
You also don't have a commercially viable business if that alone is your business.
The incentive that designers and publishers have followed since the 1970s has been to create viability by crippling the core of the business (the game) to create room to sell solutions to problems they created.
The other way is to gaslight people into doing it- Rule Zero.
This is what the Boomers--including Gary, by Rob's own confessions--did to SO MANY PEOPLE.
If they had bothered to _learn and master_ the game they played, they would see that there was no need to Buy The Widgets.
In light of GW Doing A Dumb, time to talk about IP Forking
There are two forms of forking. We'll call them Formal and Informal.
Formal Forking is obvious. This is Warcraft, Starcraft, Helldivers, Pathfinder, and several other IPs in Tabletop and Vidya- the Retroclone movement was all about this.