I say let The Forks and the Exchange (and Richardson lol) continue their growth and do what is working and on the horizon over there.
If the arena needs to be replaced in 20 years or less, start planning with Portage Place now. Tear down the Air Canada building which is awful and build the new arena on that block. It's larger N-S, same E-W but I would shut down Carlton and build over it, integrating the arena into the Free Press building similar to what the Detroit Lions did with
Ford Field*. With Portage Place presumably opening up the streets again (even if this current plan doesn't happen) at some point, losing that block of Carlton would be negligible. Then TNS stays close to their current empire, and all the hospitality and businesses that have been built up around it wouldn't have to shut down. Because let's face it, even 20 years from now if we have a new arena district near The Forks, the majority of the restaurants and bars in the SHED would not survive having no events.
*Ford Field integrated a huge old department store warehouse into the building, it's exposed into the dome. Most of the suites and club bars are actually in the old warehouse, it's very cool. Ironically, the store was called Hudson's and it was on Woodward Ave.