The majority of people who fill MLB venues with over 30,000 capacity are not die hard fans that will watch no matter what. Those kind of attendances are inflated by people who have a choice of what they spend their disposable income on, where baseball is competing with other entertainment.
This is especially true in North America where you get a lot more families in the crowd and it’s a whole entertainment package. When the team isn’t putting on a show (e.g. they are losing and don’t have any famous players/stars) that entertainment dollar gets spent on a film, or a theme park, or a basketball game etc.
So we shouldn’t be worried in year one (or 2 if you count last year) of a rebuild as when the team is successful again a Jays ticket will again be a sought after entertainment spend.
The closed system of no relegation, revenue sharing, and higher draft picks for the worst teams should also ensure no team ever goes through a 8-10 year rebuild where they bottom out for the full period. IMO that’s the kind of length that could potentially kill a franchise and force a relocation.