So the alternative is opaque types?
Powershell tried adding structured types for the output, and it has some benefits, but now your pipeline composition options are severely limited. Only the tools that understand the output structure of the prior step are options. What if you have a new structure output type? You now need all new tools to do anything with it.
I don't say text is an ideal format by any means, but it's a _universal_ format. That unviversality is necessary to be useful long term. Unix just picked text since it was easy for a human to look at and reason about.
Or are you suggesting monolithic tools instead of composable ones? Tomorrow I have a need you haven't thought of. Now your tool is useless to me and I have no way to do what I need. Modular composition isn't just a Unix philosophy, it's a basic development principle because it puts the power in the hands of the user to create almost anything they can think of, rather than restricting it to only what the creator thought of and implemented.
I'm not really sure what you're arguing is "terrible" about the solution. If you don't like the hazards and difficulty of dealing with something like your colon separator being in the data too, let me introduce you to data handling 101. It doesn't matter what format you put data in, the same problem exists. Even raw binary needs a delineator between elements, and eventually that delineator is going to need to be in the data itself somewhere. Unless you suggest we shouldn't be able to represent all data? It's the reason virtually every language supports escaping special characters (E.g. JSON's backslash escaping of quotes).