Well, firstly, understand that you are dialoguing here with a former seminarian. So I understand a little something about discernment and what people go through.
That said, I do think that there is a problem in our day in that most vocations are discerned later in life. This is the main thrust of what I was suggesting. It doesn’t matter whether the vocation is priesthood, religious life, diaconate, or even marriage. Now, that isn’t entirely a “bad” thing, but it does present certain disadvantages.
The reality of the permanent diaconate is that it is, mainly, seen as a “married man’s vocation.” One is left to wonder, therefore, if some of these married men had been open to discerning the possibility of priesthood when younger whether they might have been willing to become priests. (And, from the other side of things, there ARE young men - I know them - who have given the priesthood some thought, and sincerely believed, “If only I could get married AND be a priest,” who ended up leaving the seminary and finding a wife. Not to mention the many men who have left the priesthood to get married.) Therefore, I do believe that the question posed by the OP is legitimate for consideration, at least.
Given, the two vocations are distinct. But, perhaps, that distinction isn’t well enough defined. Why, for instance, aren’t there more permanent deacons who are younger, single men? Typically, they are not even recuited, it would seem; the assumtion being that they ought to just, “Go all the way.”
You are correct, therefore, that there are men who are rightly called to both marriage and the diaconate. But, how many of THOSE men would continue on to the priesthood, if the option were available? In some cases, they are simply, “Going as far as they can go,” with things the way that they are.
My question remains, then, as to whether some of them may have been more open to discerning a priestly call had they considered it earlier in life, before marriage. That isn’t a criticism or challenge of where they are in life and vocation now, or even genuine previous discernment, but a mere sincere pondering, asking “What if?”