Slide toppers don't stretch. Even if they did, the spring would just tighten it back up
Depending on the mfg. they can stretch. The spring or set of springs (some toppers have 2 springs) can sometimes not roll it back up tight enough. The springs can also malfunction and not work as well as new.
springs suffer from wear and corrosion being exposed to the elements.
fabric stretch in slide toppers"just close the slide and the topper will eject the snow, leaves, birds, etc. over the edge as the slide goes in"
Sometimes with no wind when snow falls straight down, it will shed by the topper.
However, sometimes not and was often the case. I was a seasonal for 17 years and everyone eventually got rid of the toppers that camped during the winter months due to snow being blown under the topper by the wind. The snow got trapped under the fabric then turned to ice as it began to melt. It would freeze as nighttime temps dropped. Many of us closed our slides at night to keep us warmer, so it became a pain to have toppers in snowy conditions. It was just too cold to have to deal with one more thing to do. Everyone switched to a home-made plywood encased topper (complete cover with sides) :)