I know it's hard to get your head around, but the tapeworms where you see the segments in the sheep's poop (Monezia expansa) are generally not really damaging to the sheep and consequently many wormers do not address them.
The tapeworms which can really harm your sheep are the ones where the segments are in dogs' poop, and cannot be treated in the sheep; they must be treated in the dog.
No your other sheep will not catch the tapeworms directly from the sheep poop with tapeworm segments in it. All tapeworms need a main host and an intermediate host. With the one where the segments are in the sheep's poop, the intermediate host is a soil mite. They eat the eggs from the sheep's poop, which leave oocysts in the body of the mite. The sheep eat the mites (accidentally one presumes, on grass they are eating on purpose) and the oocysts continue their lifecycle in the sheep's guts.
Does continually worming your sheep reduce the amount of sheep tapeworm in your pasture? I don't know that it would, really, unless you have no other ruminants (domestic or wild) grazing your pastures. Like visiting deer, for instance, which would then perpetuate the lifecycle even if all your sheep were wormed constantly.