If you were Ross Stewart, with his past few years of injuries, would you risk yourself at the tail end of the season with no contract on the table? What probability would you put on him picking up yet another injury that keeps him out long term, risking him not getting a contract offer over the summer and leaving him without any income / with reduced income? Obviously its a complete unknown - but i wouldn't put money any meaningful money on him not getting injured before the summer.
If saints want to keep him going with good form and full commitment for the promotion chase, they need to give him an extension for at least another year - otherwise they're asking him to risk his livelihood to potentially get us promoted and not even have a contract for next season to enjoy the spoils. If we don't, its probably worth more to him to take it easy and show he can stay fit, than it is to break himself chasing a few goals - his next contract will be tied to whether clubs think he can stay fit at least as much as it will his goalscoring prowess - everyone knows he's a good striker for this level.