Is it possible that it was a different John Smith who married Isabel Webster in 1733? 
There is a burial at Melling, 28 October 1768, of an Isabel Smith of Arkholme
https://www.freereg.org.uk/search_records/5fd1f343f493fd4c3b5e1190
PR here
https://www.familysearch.org/ark:/61903/3:1:S3HY-DBR7-FB2
I guess it would make her in her mid 70s by then.
I've been wondering this. It's absolutely a possibility.
The John SMITH who married Isabel WEBSTER in 1733 was from Arkholme. So, parish wise, it's a fit. That doesn't mean it's the same man, though. They could have been cousins (or unconnected John Smiths).
The 1733 marriage and the 1740 marriage are close enough in time to plausibly be the same groom.
I've, so far, only found one child for John and Isabel SMITH. I'd expect to find further children unless a spouse died or if the couple moved away. They are not the only two reasons why no further children could be born, though.
The burial you've highlighted is very interesting. Thank you for this.
I'm going to reassess the first marriage. I really am wondering if this is just a completely separate John SMITH.
This is the difficulty of researching someone with the name John SMITH. The time period doesn't help either.
Rowan Tree 🌳