You would do better to look at preposition and noun together; ἐπὶ γήρως means “in old age.” This is ἐπὶ + gen. meaning “in the time of,” but that is not the way to say it in English.
Also, why do you think that τὸν θρέψαντα is plural?
I think you will unlock the sentence now, and it will make good sense and good English.
So, when you see the list of meanings that a preposition has, and none of them work very well with a noun, do you use part of a meaning? For example, “in” instead of “in the time of” I am trying to figure out where to draw the line.
I am not sure it is helpful to introduce the idea of “moral force” here. Lukas is struggling not so much with nuances of meaning as understanding the syntax.
In the sentence you refer to I dont think there is an idea of moral force nor do I think that M. 's alternative translation is intended to highlight this. The second version is simply a smoother version of the literal and clunky “it is necessary”.
It being necessary to obey the wise, they pay heed to the fools.
(Or Although they ought to obey the wise, they pay heed to the fools.)".
Interestingly LSJ says in the entry for δεῖ: “Impers. from δέω (A), there is need (the sense of moral obligation, prop. belonging to χρή, is later, S. Ph. 583, etc.):”
Edit: Incidentally I sent you a private message some time ago which is still in my “out box” did you not receive it?
So in “ἐπὶ γήρως” γήρως is genitive so you can rule out any other use of ἐπί. Notice that M. says on p. 72 where he lists various prepositions “The definitions are only rough indications of some of the most common uses of the various prepositions”.
In time with experience you will learn the range of each preposition. The definitions M. gives are a starting point.
Thus ἐπί with the genitive means something to do with time. From the rest of the clause “δέον τρέφειν τὸν θρέψαντα” you can see that τρέφειν and θρέψαντα are from the same verb, the first form is a present infinitive, the second is an aorist participle accusative m. sing. which is the direct object of τρέφειν. You will need to work out whether ἐπὶ γήρως goes with τρέφειν or τὸν θρέψαντα. As the sense of the sentence emerges you will realise it has to go with τρέφειν. Turning to the second part “οἱ πολλοὶ οὐκ ἐθέλουσιν” you can see that here we have a subject οἱ πολλοὶ and a 3rd person verb ἐθέλουσιν with a negative οὐκ. Finally looking at the whole sentence we can see an impersonal verb in the first clause and a second clause which has no grammatical connection with the first. So we can see that this is an accusative absolute construction.
How to translate? M. on page 221 glosses τρέφω as bring up, rear (children); nourish, maintain; cherish. So Children are reared but the old are not reared, they are maintained or “cared for”. The Greek is using the same verb in two forms, in English we might say “care for the one who cared”. When do we care for those who cared (for us)? ἐπὶ γήρως could mean “in old age”, that seems to fit. “It is necessary /we ought to care for, in (their/his/her) old age, the one who cared for (us).” The second part is the depressing truth “But many do not wish (to do this)”.
You might choose other words to translate “τρέφειν τὸν θρέψαντα ἐπὶ γήρως” for example “to care for the the one who brought us up”. You might want a more a more Shakespearean version of “οἱ πολλοὶ”. But you can only move onto the question of translation when you have thoroughly understood the sentence in Greek.
Perhaps an over long post but to make progress you have to get out of the habit of rushing to translate. Try to find the meaning in Greek by explaining to yourself the function of each word in Greek. Understand the syntax then think about the translation.