I’d say changing a 5800x3d to any current CPU would be a downgrade for MSFS.
If you are only getting the CPU for MSFS the 5800x3d is the way to go. If you have other apps (especially non-game apps) that need the performance the 13700K might be the way to go.
In the future Intel may introduce faster CPUs that fit the same motherboards are the 13700K, but AMD has switched sockets with their newer CPUs so the 5800x3d can’t be upgraded with the same motherboard.
And of course there all always new CPUs on the horizon. The AMD 7800x3d is likely to appear in the next few months.