You're right, that's the obvious question and there's no right answer.
As with almost every question that's asked on these forums the first words of the answer should always be, "It depends."
Advantages of the one-box solution -- Simplicity, takes up less space and requires at least one fewer cable. In theory, the manufacturer has taken some care to match the two components and the total manufacturing cost is less than for two boxes. Disadvantages -- You already know the major one, if one of the components fails you're out of music. In addition, transports aren't being improved since the need for that technology is disappearing, while DACs are improving at a rapid rate, for now. It's often very difficult to find a replacement transport mechanism for a device only a few years old.
Advantages of the two-box solution -- Flexibility, being able to use the DAC for multiple sources like streaming, TV audio, etc. If one component fails you have to replace only it. Take advantage of increasingly good digital to analog conversion without having to replace your disc spinner, though there are one-box devices that have digital inputs for use with sources other than the transport.
Disadvantages -- Complexity, twice the boxes and cables. Requires a little more knowledge to integrate properly but that's an issue only once.
An important underlying consideration is that disc spinners of all kinds are an old technology as technologies go and some companies, like OPPO, are already discontinuing manufacturing of players that use them. That will only accelerate and if it survives at all it will be as a niche market with the resulting increase in cost. In a few years it will be as hard to find a transport of any kind as it is to find a SCSI drive for a computer now.
