That seems like a lot of assumptions. How would the Rebel take a "back seat" to the R5 and R6? From a production standpoint, they are completely different lines and freeing up the production of crop sensor DSLR Rebel production lines would be unlikely to have any impact on the production of mirrorless full frame cameras. Are you suggesting that Canon would delay the T8i because they have a large inventory of other Rebels? That's unlikely. What evidence is there that they have a large inventory of Rebels on hand? Even if they did, there a lot of ways to reduce inventory without delaying a new release.
The "game" over the next six months is going to be getting supply chain and production levels back on track, restoring profitability and hitting (reduced) sales targets. The effects of putting world economies on hold will be felt well into 2021. Canon's bottom line is a lot more dependent on Rebels than on the R series, so I doubt they made this decision lightly.