Marxian economics was always dismissive of mainstream economics, but it actually had a sizeable following and a certain level of academic respect in economic circles extending into the early and middle 20th century.

Marx had great respect for both Smith and Ricardo; he had a beef with the vulgar economists who sought to mystify the actual relations of production, workplace totalitarianism, people's material condition etc.

It was only with later developments that rejecting Marxism went from a majority view to a near-unanimous view in economics

Marxian economics are fundamentally orthogonal to the concerns of the modern economist. It asks different questions. There are a few mainstream economists who dabbled in Marxian thought, such as Samuelson, but they're generally the exception.

The modern articles from modern economists like Tyler Cowen or Brad DeLong or whatever are generally exasperatingly ignorant and uncharitable.

Marx was drafting the overall systemic internal movements of capital; they're not arguments you can make from positivist science alone. What he definitely wasn't doing was designing a rigorous mathematical system that could be applied ala reform to improve efficiency, welfare or whatever.

reserve army of the unemployed

umm...

the absence of declines in real wages

Marx's immiseration thesis was relative, not absolute; arguably it stems from his theory of alienation. The more capital the worker produces the more capital the bourgeois possess by which to direct the worker and reproduce alienated life-activity.

We saw in Part IV., when analysing the production of relative surplus-value: within the capitalist system all methods for raising the social productiveness of labour are brought about at the cost of the individual labourer; all means for the development of production transform themselves into means of domination over, and exploitation of, the producers; they mutilate the labourer into a fragment of a man, degrade him to the level of an appendage of a machine, destroy every remnant of charm in his work and turn it into a hated toil; they estrange from him the intellectual potentialities of the labour process in the same proportion as science is incorporated in it as an independent power; they distort the conditions under which he works, subject him during the labour process to a despotism the more hateful for its meanness; they transform his life-time into working-time, and drag his wife and child beneath the wheels of the Juggernaut of capital. But all methods for the production of surplus-value are at the same time methods of accumulation; and every extension of accumulation becomes again a means for the development of those methods. It follows therefore that in proportion as capital accumulates, the lot of the labourer, be his payment high or low, must grow worse.

Marx thought that an increasing proportion of the middle class would sink into the pool of workers at the bottom as fixed capital appropriated the knowledge and skills of the worker. This is to a large extent happening right now; the gap between skilled and unskilled workers is increasing.

So he made no claim that real wages would decrease; he was aware of the revolutionary power and the radical improvements in material welfare that accompanied capitalism.

the absence of declines in... profit rates

There are two main interpretations of the law of the tendency for the rate of profit to fall. One is that it's a long term secular downward trend. The second is that it's a theory of business cycles; aka. the assertion that business cycles are an irremediable, inbuilt function of capitalist production.

i. is arguable if you define profit in the Marxian sense. ii. is an entirely tenable argument.

the abject failures of socialist economies founded upon Marxist theory

Lenin openly admitted that the Soviet Union was state capitalist, which it was. He speculated that revolution would first take place in more advance countries, such as England or Germany, and that the full transition to socialism would require waiting for this to happen.

I don't see how this has anything to do with the Marxist theory. Socialism was basically abandoned by 1918.

calculation problem, and Okishio's theorem

Most socialist theories don't involve central planning. Okishio's theorem has been addressed, but that's a long foray into another world.

critiques of Marxist epistemology and methodology (such as Popper and Kolakowski's expositions of its rampant pseudoscience and dogmatism).

Popper was generous enough to extend his critique to marginalist economics as well.

From the Stanford philosophy page on economics:

Given Popper's falsificationism, there seems little hope of understanding how extreme simplifications can be legitimate or how current economic practice could be scientifically reputable. Specific economic theories are rarely logically falsifiable. When they are, the widespread acceptance of Friedman's methodological views insures that they are not subjected to serious test.

A strict reading of Popper implies economics is not adequately epistemologically grounded. That's not to say it's useless or whatever, but economists and philosophers ought to adopt and continue a particularly rigorous critique.