Generally speaking, I think the outlook for this business is fine. It's no better, no worse than others. I think there's opportunity in areas outside traditional coin business, like drop-off and commercial accounts; they are more labor intensive, but they certainly exist.
As far as business value declining over time, that's an interesting discussion. First, the addition of a competitor will certainly diminish the value of an enterprise. There's a finite number of customers in your service area and having another store is bad news and it's out of your control.
IMO, the other reason businesses decline in value is owners don't put money back into them. It's not just machines, either. It's painting, fixtures, HVAC, infrastructure, you name it. Laundries get the holy hell beat out of them every day. Successful long-term owners stay on top of those things and factor them into their budgets/model. Rookies get intoxicated by the pro-formas they see, which exclude capital improvements and one time expenses, like replacing AC units, signs, you get the picture. They get five years in and they have a store that is run down, needs a lot of love and money, yet the owner is still expecting to get a generous multiple and can't figure out why nobody is interested in buying their store. So yes, the value of the business will diminish over time, but there's usually a good reason why. Most of the stores for sale I've seen fall into this category. My advice is to budget at least 10% of your revenue for capital improvements and stay on top of things. If you can't make the numbers work adding this in, you're doomed, at least in my opinion.
Best of luck in whatever direction you go. I think this is a pretty cool business. There are many ways you can be successful, plus it can be as structured or flexible as you want. You can't say that about a lot of business opportunities.