The argument that sways Kang Jaehyuk is, in the end, Haru's: their BATNA is just pretty damn good and they don't care that much. Sure, the money they could make being top-rated espers is not the same as what the chairman of Kang Moon-il group does but it's still kind of nuts, and neither of them is particularly materialistic. Kang Jaehyuk hems and haws and tries to be a pest, and Nam Hee-seo wrangles him with ease and practice, and in the end Jaeha and Haru get what they want; Kang Jaehyuk's preferred timeline for more heirs is "as soon as possible" (especially due to the risky nature of their job) but a time horizon of within five years is acceptable and Jaeha and Haru think they can work with that.
And, of course, they get to have their autumn wedding in British Columbia. Which they politely invite Kang Jaehyuk to, and which he politely declines attending, citing his age (he's not even 70) and how he's very busy (he sure has time for mixers and the like) and how it's so far away (even though Jaeha is getting all of the guests complimentary teleports to and from the venue), but at least he gets them a very extravagant gift in the form of a gorgeous beachfront summer house in the Maldives.