Adding to NolanHerring's thoughts; as we've seen with previous generations of new WiFi technology, the gains will be accrued, over time, as client devices are replaced: new WiFi6 compatible devices will directly see the benefits themselves - and non-WiF6 devices will be rewarded with extra airtime, freed up by the newer devices transferring their data more quickly.
It's also likely that WiFi6 will offer greater capacity. Given that many WiFi solutions are replaced simply to support greater numbers of clients and/or more intensive applications, MR55 should be more scaleable, long-term.
Also remember that buying newer tech always has another potential longevity advantage. MR55 will, almost inevitably, be fully supported, beyond the End of Life of MR52/3. (having said that - remember Meraki supports products for 7 years beyond End of Sale anywau - longer than anyone else in the industry, AFAIK)