Yes. Xanathar guide recuperation rules that override the phb rules allows for very slow regeneration of attribute losses.
Ask your DM if they are used for your campaign.
What page is that on? I can't seem to find the rule you're referencing.
That factor does have to be weighed against the question of where is the fun in playing a dribbling vegetable for the duration of the time it takes to complete a more extended quest?
True enough, but then we start going down the whole rabbit hole of "If something detrimental happens to a PC, then the DM shouldn't have let it happen without providing a quick means of countering said thing"...which leads to "..and if we're gonna do that, then why bother with ANYTHING bad happening to the PC in the first place?". Slippery slope, IMNSHO.
Yeah, but there's a difference between something bad happening to the PC and something bad happening to the player. If the PC is reduced to a vegetative state, and the player has to basically wait around twiddling his thumbs while the party goes on an extended quest to restore him, that's in some ways worse than the PC just being outright killed.
As you say there are other options, and in this case it sounds like the players found one of their own - polymorphing the PC into a beast to allow the character's continued participation, at least in combats - but anything which results in a player feeling unable to contribute effectively during a session is not going to enhance the fun.
I thought the rule was that if ANY ability reaches 0, the character dies. Is that not a rule in 5E?
Interesting. So it depends on the monster, or maybe the ability. With the Shadow, for comparison, draining a character's ability score to 0 (in this case, Strength) results in death.Not in 5E. The intellect devourer's statblock states that a character with Int 0 is stunned until they regain at least one point of Intelligence. So can't take actions/reactions, auto fails Dexterity and Strength checks, and attacks vs. stunned creatures have advantage. Basically useless until the Intelligence is brought up above zero.
That factor does have to be weighed against the question of where is the fun in playing a dribbling vegetable for the duration of the time it takes to complete a more extended quest?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.