In many cases it's less a concern of the client not paying (I've literally never had a client do that once I broke out of the <$100/hr market) so much as that a

In many cases it's less a concern of the client not paying (I've literally never had a client do that once I broke out of the <$100/hr market) so much as that an upfront payment is a much stronger signal of commitment to the project. In my experience, clients who haven't yet paid anything are less invested in the process and more likely to delay communication/deliverables. It's also a consequence of occasionally doing work with startups who, while they have every intention of paying, don't always have rigorous financial controls in place and sometimes end up running into cashflow problems. I'd flip your analysis on its head: if there's a consultant you're not comfortable with paying upfront, you probably don't want to hire them.