The $300 LM deposit must generate a 7% return just to cover the current cost of the magazine and contribute nothing to other expenses

At a 0% return, the current cost of the magazine is covered for 15 years.

Assuming a reasonable 4% return, the current cost of the magazine would be covered for about 22 years.

Considering the age and physical condition of the average LM, I think 22 years covers a lifetime.