That might work well in a game like fate where everything is getting hit because there's an "8 goblins harrying you" scene aspect, unless they are physically blocking you from progressing they aren't really doing anything mechanically to impede your goals with 7 hp & 15 ac... a party of 4 can probably kill them in 2-3 rounds while moving their full speed.Monsters don't need to be able to threaten your life to be a challenge. All they need to do is threaten your goals.
For example, 8 goblins who interrupt a chase through narrow passages to try to delay you may not be a lethal threat, but it can still be challenging to defeat them fast enough to not lose ground on your quarry. If those Goblins take the Dodge action and make smart use of full cover, killing them quickly might actually be difficult, even for a mid-level party.
Lets be honest though & admit before anyone goes there.... the thread is talking about how old iconic monsters no longer fit the roles they once were able to fill. Making up some new/interesting swarm mechanics for a custom swarm monster doesn't change that.