The problem here is that what you’re describing isn’t JTBD. You’re describing a variant of ODI (Outcome Driven Innovation).
You’re right. Designing for outcomes is a problem. Outcomes are the result of an action. The problem? If that action changes in the future, those outcomes you previously measured are all gone.
Old way = buy lawnmower and take care of the lawn myself
New way = pay for a lawn care service
Now, if you measured all those “outcomes” associated with owning and using a lawnmower, well, you’ve missed the opportunity to create a new service (lawn care)
If you’re still curious for more, check out jtbd.info.