Honestly.... I really dont think he needs to even be an NHL Dman for the Avs to win the cup.
I believe this team right now(Well, not the injured team the Avs are icing at this exact moment but you get what I mean), with one or two minor adjustments, is capable of winning the cup. Those minor adjustments being one more Top 6 forward and one more 4/5 Defensive option to replace Graves or Cole(Or at least make these guys way more accountable). And I actually think Rosen and/or Timmins could replace the Defensive hole internally as soon as December or January if they ever get a chance to prove it.
Now, all of that said it would still be absolutely huge for this franchise if Byram can become a Top pairing Dman for us. Cale is going to be Cale, at this point he's probably well on his way to being a future Norris contender and bonafide #1D of The Top 10 in the league variety. But if Byram could also develop into a legit Top pairing guy, or hell even a real good #3, to match alongside Makar and have those two guys playing 25 mintues a night for the Avs for the next decade, it gives us a really good chance to extend the cup window from just 3-4 years like it is right now to potentially having something like what Pittsburgh, Boston, or Washington have had now where you can be a threat to win the cup basically every year for over a decade.
That's what makes Byram a key piece IMO. Extending our cup window beyond the current Mack contract. Without Byram as a legit stud you probably only have 3-4 shots at winning it all. With Byram as a star player alongside Cale, even when Mack gets his huge raise so long as you have a core of Byram/Cale/Sam/Mack/Mikko... you should be able to build a supporting cast good enough to contend for a cup for a long time.