Is it because Chamberlain makes the best for their own branded models? The reason that I ask is that many (if not all) Craftsman models are made by Chamberlain (IIRC). FWIW, my Craftsman has been running 20 years (chain drive) and has had the plastic worm gear changed one time.
If you hadn't said you'd had that opener for 20 years, I'd ask if it was a new change in manufacturer for Sears...BUT, it may be model specific. At one time, Genie made the openers for Craftsman...or at least some of them.
I see Sears has slightly improved the warranty on their openers...
Garage Door Openers
90-DAY IN-HOME FULL WARRANTY ON PRODUCT
For 90 days from the date of purchase, this product will be repaired, free of charge, if defective in
material or workmanship.
ADDITIONAL LIMITED WARRANTY ON PARTS
From the 91st day through 1 year from the date of purchase, replacement parts for any defective
parts on this product will be furnished, free of charge. Any costs for professional installation are
not included.
ADDITIONAL LIMITED WARRANTY ON MOTOR
From the 91st day through 5 years from the date of purchase, if the motor on this product is
defective, a replacement motor will be furnished free of charge. Any costs for professional
installation are not included.
The higher end Chamberlain openers carry a better warranty:
Manufacturer Warranty : Lifetime Motor warranty, 10 year belt warranty, 3 year parts warranty
IMO, just the warranty difference makes branded Chamberlain openers a better value.