The thing about Melmetal is that in Go they've essentially already "broken" any valuation of catching it in sheer quantity for candy, so the only value they hold there is rolling tons for High IVs (since Melmetal is a somewhat useful Anti-Meta pick for PvP) since Mythicals can't be traded for the "Lucky Trade" feature (Pokemon reroll their IVs when traded, Luckies give them a 12/15 floor among other benefits).
What I'm getting at here is that with the way the Box works, if you have access to Home already, you already have the means to drop a ton of Melmetals in if you play regularly: 3+1 Candy per Meltan capture (not counting bonuses from Berries or an Active Mega) and Transfer with an hour of spawns every ~1-2 Minutes means that you could have a Melmetal as frequently as once every 3 days, assuming you have one worth evolving at each interval (first batch you could yield as much as 300 Candy if you Pinap and Transfer most of them with a Mega Bonus active, then the excess from batch 2 means you can probably evolve a 2nd), not accounting for the rare events where the Box gets its cooldown shortened and Shinies are added to the Pool (THOSE being event only). Shiny Meltan is effectively as rare as Shiny Melmetal because even during the featured events, the average appearance rate for a Shiny more or less matches how many Meltan you need to catch for an Evolution's worth of Candy.
If a flood is a concern, they already made Melmetal permanently available by virtue of the Gigantamax one when you transfer any Go Mon to Home (which would be a pre-requisite to a Melmetal flood anyway). I refuse to believe that the G-Max Meltdown over Steelsurge was Gamefreak thinking that is enough of a "limiting" factor to make those Melmetal safe to have available widely vs standard ones (due to not being able to add or remove the GMax factor with soup).
Melmetal takes a degree of work to train up but it's not going to be a hyper rare commodity like most Mythicals are. The Go-only evolution feels unnecessary since the only way in which it's potentially monetized is on TPC's part if one buys into a Home Pass, which they have a significantly larger number of selling points to focus on than "ruining" a Shiny Trophy. Not to mention Meltan is basically only a dex filler compared to its evolution, so the majority of people transferring one either want it for the Dex Entry or a Living Dex, and to my knowledge Melmetal tends to be limited/banned from most official formats because of that Mythical status.
The contrast to this is Gholdengo: Unlike the Free Home service, the Coin bag is refreshed by specifically interfacing with SV (so not just a paid game, but a specific one of the several on Switch) in a feature that also has benefits to the Go players (Scatterbug from the Postcard features is the only way to encounter the species or get Candy to evolve the MANY Vivilion patterns). Their design motifs and presentations of tiny Metal Gremlins that amalgamate into a much more impressive Evolution with a lot of Material is already shared, so if the concern is Melmetal's availability, the minimum is to make it available sparingly outside of Go.
tl;dr Melmetal's already more than available enough that literally every Meltan in Go evolving won't affect the "economy" enough to justify this. Just let the nuts evolve.
