Reynolds' dispute with "mechanical imitation" is aesthetic and prophetic for many reasons, but political at least for one.  Followers of Samuel Johnson's The Idler would have also understood by "mechanical imitation" the "drudgery" London's ambitious painters reviled—churning out Dutch or Venetian "views" for tasteless, cheeseparing aristocrats.[i]  Even if Reynolds' judgment was not sincere (which it surely was), his protectionist xenophobia would have already precluded him—and the "enlightened" members of Johnson's "Club"[ii]—from recognizing in the Dutch and Venetian styles the basis for a poetics akin to their philosophical moment.  Besides, for Reynolds, the extant sublimity of Michelangelo and Rafael[iii] proved once and for all that catering to the eye (or worse, the camera obscura[iv]) wasted the poetic capacities of both the imagination and the brush. 

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[i] C.f. Allen, p. 32-36.  While Reynolds seems to have escaped this chore, even Gainsborough did not, producing, with a cadre of English artists, roundel landscapes in the Venetian mode for the Foundling Hospital. 

[ii] Reynolds, Johnson, Edmund Burke, Oliver Goldsmith, Giuseppe Baretti, Adam Smith, Henry Thrale, David Garrick, and Angelica Kauffman, among others.

[iii] C.f. Reynolds, "To the Idler," no. 79, p. 129.  Reynolds appeals to these painters throughout his Discourses and his Journey to Flanders and Holland.

[iv] Another implied target of the epithet "mechanical imitation."