He’s not old, but he’s too old
29-years-old is not old. Even in NBA terms. A player that age can certainly remain productive for at least five more years, more than enough time in the prime to spend the cash or assets necessary to acquire if available.
That being said, while I wouldn’t be opposed to McDonough signing a 29-year-old in free agency this offseason, if he is going to do so, that player had better be someone that is not intended to be a core piece of the roster long-term as he will age out while the current young core will still be in the early part of their prime. Isaiah Thomas would presumably be a central part of the offense for the duration of his tenure in Phoenix (would anyone assume that he would play for the entire length of his contract with the Suns?), so losing such a key figure, especially at such an important position such as point guard, could put the presumably then winning Suns in a pickle if they don’t have a suitable replacement at that time.
Granted one could say that about any player at any position, but with point guard being such an integral cog in the running of an offense, there is no doubt that he would be difficult to replace.
If Thomas were, say, 25 (both the age he was when Phoenix signed him the first time and yet still the oldest player in the current core group), that would be one thing. He’d be young enough to potentially be a part of the core for the duration of his contract as well as at least one max extension.
At this point though, he’d maybe make it through his next contract before the team would have to seek new options (especially since his age might mean that his game was concurrently breaking down), and therefore the potential shock to the roster at that time, and even the couple of years leading up, might set the Suns back, potentially – one again – nearly blowing the roster up, if not actually totally succeeding.