I think riding alone is a great test for the horse. I work hard, especially this time of year, to get all three of our horses comfortable with riding alone. It builds their confidence and makes them a better working partner. We don't have an arena YET, so all our riding is on the driveways, pastures and dirt roads near my house.
Even when you are shoveling manure in the barn by yourself, there is risk. Two years ago, I tripped, fell and got stepped on by one of our horses, the biggest one, of course. As usual, I had my phone and called my MIL to rescue me. I broke 4 ribs, spent the night in ICU, and missed 5 days of work. It wasn't the horses fault or mine. I blame it on my bum right leg and I have worked harder on my balance since then.
I came into this life to play it hard, and though I don't want to be foolish, I plan to enjoy it. If I waited for a riding partner, I would ride so infrequently that my horses would be too out of tune and out of shape to ride at all, and I think THAT is even more dangerous.