Stroman is only 27, is a solid to very good pitcher and barring injury he should remain so for at least the next 4 or 5 seasons.
If we don't extend Stroman, the chances of us developing a pitcher as good as him when we need one (which could be as soon as next year, but is probably more likely a couple of seasons away) without overspending on a free agent (something Shatkins probably won't do) isn't very high considering how often pitching prospects fail.
Having said all that, it take both sides to agree a contract. If Stroman is asking a ridiculous salary and/or contract length, then we should trade him at the deadline for a haul.