Clarkson is 29 years old. He is on a uphill, progressed 2 years ago and this season was on a similar pace.
Briere is 35 years old. He is going down hill since 2010.
Nobody has a crystal ball here, but logic show than albeilt an injuries or a weird simultaneous decline, Clarkson still has a good 4 solid year ahead of him.
Whereas the only chance Briere help us next year is that if by magic he, at 36 years old, gains is form back from 3 years ago.
And yes , 4-5 years from now, a 5.25m$ contract for a 33-34 contract will be manageable for a powerfoward who is still probably be able to play in the top 9 albeilt a severe regression. Worse case senario, you bought him out, Toronto can afford it. It's a still overpayment, it's a risk, but there is a high probability that Clarkson gonna keep being a 25 goals powerfoward for the Leafs for a couple of other season and gonna help us being a considerably better teams.