I note that 'WorkingClassBro' is repeating one of Marx's more hilariously wrong predictions.
Capitalism is more a slur than a well-defined economic term
It's useful as an economic-term-in-historical-perspective because it isn't as though there's another word lying around to be used, and as new jargon is invariably terrible.
I like market economy, that's a term which I understand the definition of. Whenever people talk about capitalism they're usually being very political and very emotional and I'm not sure EXACTLY what it means. It seems to often be something like "United Fruits shooting union members in Latin America"
Political economy is a political subject. Any descriptive term used for the positive analysis of economic systems will inevitably be used to make normative claims. I am not exactly sure what defines a market economy, which is a term that strikes me as a shibboleth for those on the right.
It's certainly true that capitalism is an obsolete term that only has a clear meaning when we're talking about the 19th or early 20th century. Political economy was pretty cut and dry then: the only industrialized nations with functioning markets in all arenas of life were characterized by nightwatchman states. Now the most meaningful questions of political economy are not centered upon deciding whether or not markets should exist but rather what values markets should embody. Even radical Marxists agree that markets should exist: their argument is that computing power should facilitate capital flows or that worker-owned firms should be responsible for the decisions made by "capitalists". The idea of the "command economy" is defunct.
I agree that capitalism is a defunct firm from a different era but all substitutes for the term are heavily politicized. For instance, I think that "neo-liberalism" or "late capitalism" are very descriptive and useful terms but that they've been ruined because they've been used pejoratively by activists.