The US equities market value is already highly inflated. It will be coming down regardless over the next decade.
I'll take the other side of that bet if you are saying that the market will be worth less in 10 years time than it is now.
If on the other hand you are saying the market will go down sometime in the next 10 years then, we'll that's not really a statement worth making:) That's like saying I predict that it will be colder than it is today sometime in the next 10 years.
The time premium on LEAPs makes them, as I understand it, an extremely niche product to use appropriately.
Of course anyone is free to speculate, as long as you meet the option trading requirements at your brokerage.