I agree with your main point.
my rambling speculation on Pokemon and Smash below:
Smash has more leeway in terms of release windows ranging between 2 (64-Melee) and 7 years (Melee-Brawl) while Pokemon has become fairly consistent with release windows. Smash has stuck to the 1 game per console so its likely it'll stay the same for Smash 6.
N64: 1996
SSB: 1999
GC: 2001
SSBM: 2001
Wii: 2006
SSBB: 2008
Wii-U: 2012
SSB4: 2014
Switch: 2017
SSBU: 2018
With the exception of Melee, Smash games tend to come out at least a year after a new console is released. Since the N64 consoles tend to come out every 5-6 years so if this holds a new console should be out by next year (this year was the earliest a new console could've debuted) and Smash 6 would be out by 2024 at the earliest.
For Pokemon, since Gen 5 a mainline new gen Pokemon game has released every 3 years. However, their anniversaries were in 2006 and 2016. Both of those years had mainline games released as well. If Pokemon sticks to the 3 year schedule then the next gen should be in 2025. If Pokemon is to align Gen 10 with the 30th anniversary then it will release in 2026. one of these "guidelines" is bound to break.
So if Smash 6 can't come out no earlier than 2024 and Gen 10 can't come out no earlier than 2025-2026 then if Smash 6 = 2024, Gen 9 gets the Smash slot. any later then Gen 10 is likely to get the slot.
If it's Gen 9, which Pokemon might make it in Smash 6?
Based on the current Pokemon roster in Smash:
6 have been starters
1 baby
1 legendary
2 which aren't a starter, baby, or legendary.
since Brawl a starter pokemon has made it in Smash. Brawl was also the last time multiple Pokemon made a Smash game. Also Ivysaur is the only non-bipedal pokemon on the roster.
it seems likely that a bipedal starter would be the most likely pick so
Meowscarada or
Quaquaval could be added.