Why I think Buddie will go canon in season 10
First things first, Buddie is not a slow burn. It was exclusively a fanon ship up to season 7. Most intense moments between Buck and Eddie did not have explicit romantic tone. It was unusual and intriguing how tightly they wove their lives together, and while it could serve as a foundation for romance, it wasn't romantic per se.
Things changed in season 7, when Buck realized his bisexuality through his jealousy about Eddie bonding with another man. That was explicitly shown and declared throughout both seasons 7 and 8. Buck explicitly admitted to having feelings for Eddie and at the same time shut down any discussions on that as he felt he was taking too much of Eddie's time and thus hurting both him and Chris.
Eddie explicitly stated he viewed romantic partners as something enforced (by community, family, society) or as competition for his focus on parenting Chris. All his romantic pursuits ended in some kind of disaster and he's avoiding romance like plague.
Anything before season 7 is potential/possibility. Everything since season 7 is textual and explicit. Buck has romantic feelings for Eddie and doesn't pursue him because he's straight and even if he isn't, Eddie doesn't want romance. Eddie is hiding from his romantic "failures" via denial of romantic and sexual parts of himself. That is the canon at this point.
Now, what we have is a show focused on family and romance going into its 10th season (that's a long time to be around nowadays) with most main characters having established romantic partners. Athena isn't getting with anybody that soon after Bobby's death. Which leaves Buck and Eddie. Seasons 7 to 9 gave their relationship an unresolved romantic issue through Buck's suppressed feelings and Eddie's denial of his romantic needs. The last 3 seasons also tied them even tighter together, removing any love interests, separating them at work but tethering them together in their personal time, and focusing their standalone episode on being mistaken for a married couple and putting the other above their own survival.
From the business perspective, the show needs something huge. Before Bobby's death, it delivered the formation of the 118 family and establishment of exciting love stories (HenRen, Madney, Bathena). That is what kept the audience engaged and loyal. With or without Bobby, the 118 is a family, and the popular ships are well-established. The only big thing that can happen now is exciting romance for the remaining unattached leads. Based on seasons 7 to 9, it's going to be a romance BETWEEN the said leads.
Also based on the writing in the last two seasons, I suspect #10 might be the last one, so it makes sense for the show to keep Buddie as the main attraction for season 10 and maybe even use it to squeeze one more season.
