I like your take on the idea, although I think there should be a minimum that their stats can fall to – someone who is built like a mountain isn’t going to lose all that much bulk even if they stop hitting the gym in favour of hitting the books, and someone who is very sociable and outgoing and happy-go-lucky isn’t going to lose all their spirit just because they’ve started getting swole (although they might become more surly if they’re hitting the books hard at school/uni and have to study when they’d rather socialise, heheh…)
I’d say those minimums should be half of the hearthling’s maximum, rounding up – so if they were a Mind 5 and they spend all their time training Body or Spirit (or both), they won’t fall below Mind 3; but if they started out as Body 2 and spent all their time training another stat they could fall to Body 1.
Actually, I think it would be great if unused stats fell away even when there was no training taking place. It doesn’t matter if your settlers are all high in one score when you embark, if the town doesn’t challenge its citizens’ attributes then those skills will be lost/replaced over time. So, taking this idea forwards, I’d propose that instead of thinking of it as “you have to give up one area to gain in another”, it works as “if you don’t use it you lose it.” That’s a significant step away from your proposal; but it adds more of an “upkeep” element which encourages the player to build those amenities which the hearthlings need to keep their minds, bodies and spirits in top condition. Of course there’s the matter of balancing it so that it doesn’t feel like a chore, but I think this can be accomplished easily enough by having some passive boosters for each stat so the player can easily maintain their current levels; it’s training them further which would take a significant investment.
Another way to prevent the “obligatory new-Hearthling training grind” situation is to make it progressively more difficult to train each level, so you can go up to say 3 in each stat with only the most basic amenities (probably all made by the carpenter/potter/whatever the NA’s primary crafter is), but to get those stats up to 4 requires input for more classes and higher levels of those classes, and getting to level 6 requires rare materials (e.g. a shrine to increase spirit might need one of the Kobold gongs and some gold, the gym equipment for Body 6 might require varanus hide and other products associated with strength, and the Mind 6 training item might require rare tomes purchased from a trader as well as items looted from enemies), top-level craftsmen (and ideally different craftsmen for each one), and the recipes for the higher tiers might not even unlock until you have a few hearthlings trained up to those high levels as well as being a tier 2 town (so, for example, once you’ve reached tier 2 you might draw the notice of a famously strong hearthling who tells you their daily workout routine and challenges your hearthlings to train up and try to beat them in a contest of strength – with suitable rewards for winning, naturally…)
There’s a lot of potential to add to the gameplay further, and make players really think about how and why they’re investing in their hearthlings. It will definitely give players a deeper sense of attachment if they’ve helped their hearthlings train through different stats to become great at their jobs, and there’s a huge sense of accomplishment in that.