I'll admit that I prefer it when there's canon lore explaining how new races came to be on Toril if they've not been there previously, and just the fact that something is in the D&D ruleset is not enough for me without such a story (given that while the ruleset is setting-independent some elements were designed with specific, non-FR or non-Toril settings in mind). But I'm not overly fussy about what the stories are, and within reason am happy that WotC actively look for opportunities to make as many D&D races, classes, etc as possible available to players in whatever setting they prefer. Players or DMs always have the option to elect not to include new races/classes in their home games. New additions will inevitably impinge in PC games, unless they're restricted just to optional player characters, which would probably feel a bit weird to me unless it were somehow made plausible they were the only exemplar of their kind around, but I'd rather err on the side of too much inclusivity than too little.
I believe there's already a story about how Dragonborn came to Abeir-Toril when the worlds collided for 4e and (some) were left when they separated again for 5e. Being a 5e newbie, I'm not aware of what if any lore there is for the other races or the artificer class, excepting Warforged which I understand have been specific to Eberron. But as long as there are explanations then I'd be happy for anyone to appear in the game, and indeed for Larian/WotC to make up explanations if they want!
I'm more picky in other contexts, but for me FR and Toril's as a setting in which people can enjoy roleplaying games and telling the stories they want to tell trumps things like narrative plausibility that I'd prioritise more highly in other media.
