actually, ive always heard this expressed a little differently, which was miracles were needed when the existing structure was changing. in jesus' time, the jewish system was god's established means of worship and so miracles were provided to establish that gods blessing had moved to the christian congregation. once a structure is established, miracles are no longer needed.
in a similar vein, the WT points out all the semi-miraculous signs of the WT being chosen by god and a new structure replacing an old. there was 1914, clearly marking the kingdoms birth just as foretold, the incredible reversal on the long prison sentences imposed on the WT leaders, the mighty, earth-shaking 'trumpet blast' conventions from 1922-28, and of course, the miraculous growth. what? you didnt see all these miracles? where were you? well, too bad anyways, cause youre not getting any more.
mox