For Marx everything is a tendency, in his logic, we go beyond the simple positive statement and into actual historical development, consider for instance he correctly anticipated the fight over wages between the working class and the capitalists (which today precipitates in the creation of many workers' rights laws), which serves to counter-act the tendency, and a greater rate of exploitation (rather than a lower wage) is one of them. The biggest clue against your reading of Wage Labour and Capital is that Marx believed that a rise in productivity tends to a rise in wages.

Marx only identifies the introduction of machinery lowering the wage rate on the idea that with such machinery, workers are more substitutable - but as far as I know he did not account for the rising proportion of people taking up what he described as a "numerically unimportant class", "engineers, mechanics, joiners". In fact, this class of workers has not seen the same reduction or stagnation in the wage rate that Marx saw would happen with low skill workers.

"And precisely taking into account the forms of technical progress which Marx considered - that is the division of labour, capital saving, the mechanization of production and innovations - a fall in the maximum rate of profit can happen only in very particular cases, and no need arises to keep wages at a minimum in order to avoid a fall in the rate of profit."[0]

[0] https://mpra.ub.uni-muenchen.de/20976/1/MPRA_paper_20976.pdf