...And a good tenant that pays fair market price is even better.
It all comes down to 'fair market price'. If it's truly fair, than you are going to earn nothing over a long period of time and if these people are friends and family then that's one thing, but if you are treating it as a business I don't understand the rational. If these people are paying you under FMV, I'd give these people notice and start looking for a new long term tenant willing to pay you the FMV.... it shouldn't be that hard... again, if the price is truly fair.
In this case, sounds like you have a tenant for years that priced his goods and services based on cheap rent from you. That was his mistake you are are now accepting as your own. And it's the kind of mistake you'll talk about in 10 years as possibly one of the the worst business decisions you made, because it was a sizable asset that that should be compounding interest, and gaining value over time.... and instead you actually lost real value, especially when adjusted for any inflation.
So its up to you to define what is fair and reasonable and then set some boundaries. If you have to compromise some, then it's ok, I don't advocate cutting your nose to spite your face. But don't just take it because your are lazy or afraid of a little risk. Because in my mind, if these were my tenants, and they were in such bad shape they cant ever accept an increase without it dramatically affect their business, that business has a great chance of failure. And when that happens you will waste months, then possibly face eviction and losing huge.
If it were me I look hard at long at the FMV of the property, and what getting a new tenant will to pay me at least FMV will cost me.... the difference between the two... then cross reference that with how much I think these tenants are pulling my chain, and how far they could meet me in the middle... then I'd pull the trigger on the decision. If you have no hope of making money on rent, then sell it and invest in something else that has less risk.