I agree with that as far as it goes. What we have to keep in mind about the situation is that the dynamic was as follows, towards the end:
Shannon was the top chicken, Lilly was pro-Abby, though also anti-Frank, thus more in the Shannon camp. Becca was neutral to all sides and Abby was definitely in the Frank camp. As with the fact that she never called Frank dad in front of Shannon and Lilly, she also never addressed the elephant in the room that it is very unlikely that Trevor is Becca's father and thus Lilly and Becca are only half siblings. Like the dad thing, it would have disrupted the fragile power relationship in the family and shifted power more towards Frank (if only because Becca would have been anti-Shannon and neutral to Frank from that point on), which is intolerable for a person like Shannon who needs the position of power. In short, for Becca to address this circumstance would have only brought disadvantages.
Now let's imagine the hypothetical chaos if Becca were really Frank's daughter:
Becca would have gone completely openly pro-Frank and anti-Shannon, plus Becca and Abby would have had an even more sisterly relationship as a result.
But now we come to the worst case scenario, Lilly: Lilly still had her complexes about her own father at the time, so if it had come out now that Frank was Becca's father, she probably would have moved further away from her family, especially if Becca and Abby had developed a sister dynamic and she had to share Abby.
The bottom line is that you can say that would have given a series of escalations, especially of the explosive personalities that Shannon has and Lilly had
Now that Lilly's emotional wounds are healing, however, I think Becca may well discuss this with Frank at some point when the time is right. Something along the lines of "Dad, didn't you ever wonder that when Mom had me she was around 16, while when she had Lil she was around 20?"
And to be clear again, to avoid misunderstanding, I don't think Becca is Frank's daughter. Could Becca have been the result of a drunken fling in a back alley or restroom while Shannon was having a girls night out with her clique? Absolutely a realistic possibility with Shannon.