That's fair. It would be better to break it down into two parts. The in "economy" side of roblox, where the dev gets 70% or the product they sell, and the DevExchange program where devs convert their robux to real currency. So a robuxs real world value is only 35% to the devs, but Roblox covers servers, storage, cust. support, and moderation from their own share.
Personally, I think Roblox is a great entry point to game development. A great place to start building all diff. development skills. While its possible to make a whole career of it, I think many will take the skills they started learning trying to make games in Roblox and move on to more Robust( see what i did there?) and unchained engines such as Unity, Godot, or even something like GameMaker Studio.
Where I see the bulk of people making decent money on the Roblox platform is by doing commissions for assets for other devs.