No way to know, but I think the population is large enough and the sports culture significant enough that it won't be an issue. Also, the hockey team will have a 5 year head-start (minimum) to build a fanbase, and provided they don't totally stink for the entire time, they should have some entrenched fans by that point that won't necessarily abandon the team because the shiny new hoops squad comes back to town.
Of course, who knows if/when the NBA ever comes back to Seattle?
Ahem....as far as sports towns go, I'd put Detroit on par with NY, Boston, and definitely LA.