I don't use old thread. I used to and thought nothing of it. I however used to have an embroidery franchise. The training for having a franchise was about 6 weeks. 2 in their training center and about a month in the store. The one thing you heard over and over, if you have trouble stitching, check your thread. Do not use old thread. Old thread is the cause of a 1000 different issues. Humidity, heat and all kinds of things cause thread to go bad. Thread can dry rot too. It looks ok on the spool but when you sew, the thread is breaking and stitches are skipping. It isn't the needle, it isn't the machine, it is the thread. Re-threading won't fix it, you have to sometimes bite the bullet and throw away the thread or sit it on a shelf and let it be just pretty but don't sew with it.
I quilt only with embroidery thread, it is way cheaper than Aurifil. I sew usually with new cotton thread unless I'm making a garment and then I will polyester blend. I sew a lot and I don't use a 1000 different colors, I stick to basic neutral colors so it isn't a big expense to me. I will buy all the fabric I can find at a thrift store or garage sale but not the thread.