I'm not sure I follow your logic here. Can you elaborate?
For the record, there's no guarantee Covid comes back stronger in the fall. Like any RNA virus, SARS-CoV-2 mutates regularly, but most mutations aren't clinically significant. On the contrary, the bulk of mutations occur in non-coding regions of the genome, or result in a virus less fit to replicate. Moreover, multiple genes often encode virulence determinants. And unlike influenza, coronaviruses aren't known to regularly swap surface proteins between strains through antigenic shift, the mechanism usually responsible for pandemic flu.
The longer the virus replicates, the greater chance a meaningful mutation(s) occur. But the evolutionary deck is stacked against the right(wrong?) mutation(s) occurring in the right (wrong?) place(s) to create a deadlier SARS-CoV-2.
All that being said, it is theoretically possible COVID-19 could get worse with second and subsequent "waves". But it's also possible it becomes less virulent, as ultimately it's not in the virus' best interest to kill off too many hosts.
The bigger problem is adding another widespread infection to a medical system designed to function near-capacity with the existing menu of disease. We kinda lucked out COVID-19 arrived as flu was trailing off in the US. Hopefully all the NPI adopted to curb COVID-19 will curtail other infections as the next flu season approaches.