@Brice Hall, I think you're actually misunderstanding something fundamental about capital expenses. Whether the roof is 30 years old or brand new, it's losing the same amount of value over time, and (should) be accounted for in long run cash flow projections the same way. The only difference is in the current depleted value, which you need to consider in the price paid for the property.
For example, suppose that on a given house the roof costs $12,000 to replace, and is expect to last for 240 months. I therefore need to account for $50/mo in capital reserves for the roof. Similar for other major mechanical systems. These should show up in my long-run cash-flow expectations. The current age doesn't matter from a cash-flow projection standpoint.
Of course, when I buy any property, I need to consider how much depleted value the roof and other major items have, and set aside (or at least plan for) that expense right now, as part of what I consider my purchase-price to be.
I've a number of these old/cheap properties and I believe I'll come out fine - but time will tell. I just bought another one for $30k, spent another $15k to fix everything on it including new roof, and I'll be able to rent for $800 or so, expecting $400/mo NOI. That's a good deal in my book, but I'm sure others do better.
I might lose an entire year of what I can "net income" when one of these systems needs to be replaced, but that's okay. There's only one of that system in that house, and it won't break again for another 20 years.
I think it's the people buying NEW houses that are at greatest risk of misunderstanding. If a person buys a house with a new roof, is that person still deducting $50/mo out of cash flow expectations to replace it, as I've shown above? How about on the furnace?