Call me cynical, and maybe I view the game differently. But aren't rules like this just giving players tools to beat DMs over the head with?
What is the difference, from a game play perspective, between a party using the downtime in some way of their choice, and the DM rewarding that downtime in some way of the DM's choice. And the party using the downtime in a way prescribed by the rules, and the DM rewarding that downtime as prescribed by the rules?
In my mind, the only difference is consistency between games, and the restriction on the DM. As an example, if I am a DM, and you role play the learning of a skill during downtime over 15 sessions. I can award you proficiency in that skill. If the rules outline a different outcome, is that a net positive? In theory, the DM could no longer award what they felt was the proper reward, but instead has to reward what the designers designate as the reward. If the DM doesn't, the player may say "but sir, the rules on page 691 of the PHB say I get this."
So these rules are perplexing in their purpose. Is the purpose to force consistancy? Is it to make DMing easier? What is the net gain of limiting options by rule? And if the rules are so vague as to not limit options, what purpose do they serve at all?