Well, while there may not be a default time or cost to work with for spell creation, there are two other considerations. One, the spell creation guidelines (we know WotC breaks their own rules here, buffing damage and nerfing healing, but that's a whole other conversation) and of course, what the DM will allow. Just like any other option, what you have access to is decided before play begins, with the DM having a pretty strong say. Asking for a spell all your own is right up there (IMO) with asking "Hey can I play a Bloodhunter or a Gunslinger?".
So most likely, the only way you'd get a WeWinEverything spell is if your DM is asleep at the wheel, lol.
Besides, we already have ways to win D&D with stuff WotC published, like Wish/Simulacrum "Snow Cone Wish Factories", 18th level Wizards using Spell Mastery on Silvery Barbs and using Shapechange to become a Marilith to gain a free reaction on each turn (!), using Wall of Stone and Fabricate to build massive fortifications in days, or using Teleport Circle to upend world trade, among other things.