To be fair to Kelvin, it was a reasonable opinion at the time:

1) Half of the aviation pioneers died trying.

2) The Wright brothers were truly overachievers, devising a control system, early aviation engine (1903) and wind tunnel.

3) Producing light-weight, reliable and powerful aviation engines took over 20 years. The Atlantic crossing only became survivable after the Wright Brothers made the Whirlwind J-5 radial engine (1925):

https://en.wikipedia.org/wiki/Wright_R-790_Whirlwind

For millennia, the pursuit of flying was about as successful as perpetual motion or alchemy.

What's interesting is that it was possible to make gliders for thousands of years. The secret ingredient is to glue fabric along the frame, not sew.