My prediction is that the overall volume (at least seat-miles) of business travel will hit the 2019 level fairly quickly (maybe 2022-23). But the composition and nature of that travel will change. Travel for meeting a client will probably never recover: the clients know they're ultimately footing the bill for travel and they have a lot more space to just say "this could be a zoom". For sales, the value of travel is also more or less in inverse proportion to how often the people you're selling to are in the office (it'd be pretty creepy to show up at the front door of their home...).
So miles/segments (i.e. time spent) for the most frequent of frequent travelers will, at the median, pretty dramatically decline relative to 2019. But a shift to more distributed teams implies more internal travel, especially in higher-income professions (consider software development). A couple of days in the office every fortnight for some; 1 week in 4 for others; and even for more clerical jobs that could absolutely be done 100% remote, 2-4 offsites a year. Everybody who's working what would have been an office job in 2019 will probably end up doing some business travel. That business travel will look a lot more like leisure travel in terms of scheduling, too (which will hide the rebound), and some of it might not even be OPM (if you're earning a superstar metro salary but living most of the time at what was your summer house, you're probably not expensing the flight).
That means a lot fewer high-tier elites and a lot more low-tier elites (and possibly even tiers below the current AA Gold/DL & UA Silver tiers), and the benefits that move the needle might be different (if the mix is more people who buy up, are complimentary upgrades a useful benefit?).