Soft potatoes are safe to eat as long as they have no mold or any bad smell to them.
Whenever I have potatoes that have gone soft I will usually boil the soft potatoes to make mashed potatoes or use them in soups and stews.
Potatoes do start to soften up and shrivel up as they go bad but as long as they smell good and have no mold growth or anything else on them the soft potatoes are just fine.
Also if you notice any sprouts on the soft potatoes you can remove them and still eat the potato.
Even if the potato has some bad spots to it you can remove the bad spots from the potato and still eat the rest of the potato.
If you notice your potatoes are going soft you need to use them up soon because they will turn the other potatoes bad.