They would still harm it in a way but in the sense of not advancing it rather than incurring the anger of two Great Houses as they did in OTL. This scenario might spare Aegon the wrath of Houses Tyrell and Tully*, but it also does nothing to support his political reforms. Aegon relied on marital and future blood relations with the great houses to aid him in passing his reforms, but if by some stroke of extremely bad luck, both Luthor Tyrell and Celia Tully die without issue, the marriages would not have served his purposes since those alliances would be all but dead. All that would tie the crown to those houses at this point is mutual regard, and little more. Perhaps he might hope to count on whatever personal relationships Jaehaerys and Shaera might have cultivated with their late spouses’ families while the marriages lasted but that’s not an assured source of support. Which is why Aegon would want to make other dynastic matches for his children (Jon Arryn, for example, for Shaera, and Olenna Redwyne for Jaehaerys to make up for the broken betrothal with Daeron) so he could count on lordly support for his reforms.
(*I say might because neither Jaehaerys or Shaera was exactly the soul of
responsibility and duty. I can see them following the model of Rhaenyra
and Daemon the Rogue Prince and not waiting an appropriate amount of
time after the death of their spouses to get married if they anticipate that Aegon would look to other dynastic matches and look to forestall that by presenting their marriage as a fait accompli as they did in OTL. Needless to say, that would not sit very well with the
Tyrells and the Tullys, and might even give life to rumors of a previous affair
between the siblings.
)









