A proper incinerator doesn't release toxic gases and generates electricity so perhaps fewer fossil fuels can be burnt while it's running so it's not quite as bad as how you're phrasing it either.
I don't claim to know what's best out of recycle, incinerate, landfill and biodegrade. To me none of them seems obviously bad. Various calculations would need to be done.
"Waste-to-energy plants cause less air pollution than coal plants, but more than natural gas plants.[2] At the same time, it is carbon-negative: processing waste into fuel releases considerably less carbon and methane into the air than having waste decay away in landfills or bodies of water."
"Burning municipal waste does produce significant amounts of dioxin and furan emissions[4] to the atmosphere as compared to the smaller amounts produced by burning coal or natural gas. Dioxins and furans are considered by many to be serious health hazards. However, advances in emission control designs and very stringent new governmental regulations, as well as public opposition to municipal waste incinerators, have caused large reductions in the amount of dioxins and furans produced by waste-to-energy plants."