The different companies buying ads are not interchangeable. Brands, who typically have the biggest budgets, all care about brand management (where their brand is shown, what it's shown next to, before and after), which can't be done effectively now that the staff that handles these things has been gutted. There's no way to get those advertisers back without rebuilding that part of the business and there's no other type of advertiser that can step in to fill the size of that gap.

If anything, the reduced quality of the performance marketers who move in to buy ads for scammy weight loss, skin care, get rich quick offers are going to drive away any big brand advertisers who are actually trying to stay on Twitter and weather the storm.