...I am quite happy for my players to set out to get training in new skills in-game, but those skills are specific, not general. For example, if a character without Survival as a proficiency wants to learn to track better they can, and assuming they spend the appropriate time learning from a knowledgeable tutor, they'd get to be able to treat Survival as a Proficient skill — but ONLY for tracking. Not for any of the other skills that use Survival as their base.
Likewise for climbing, swimming, poetry, and so forth.
