You're locking in 6% for 30 years, inflation is going to vary.
Or, assume rates go up. Then you've borrowed money at 6% in a 10 or 12% inflation environment. That's not bad, either.
Personally, I'm not sure that I like "load up on debt" as a strategy, even if it makes sense financially. But it can make sense financially.
Note well: The hidden assumption here is that interest rates track reasonably well with inflation, and also that your income tracks reasonably well with inflation. The first assumption is probably reasonably solid. The second one... your mileage may vary.