How hard is it to migrate a design from 40nm to 28nm? Can this be automated?
Did you throw RTL at a layout engine and let it figure it out? Pretty damn close to automated and could get to automated with a little upfront elbow grease by a company specializing in such things.
Heavy analog design? It's going to be a lot more work.
I.e. the traces would have essentially the same standing-wave tuning requirements when modelled as waveguides; would catch a harmonic of the original frequency when acting as antennae; etc.
Additionally, you very, very rarely have the antenna on chip, and the analog bits even for RF are more signal conditioning that isn't typically modeled like waveguides, but instead more like those old analog plug board computers, simply integrated onto a chip.
I didn't mean that there would be components intentionally serving as antennae in a design; more that you might be choosing analog trace lengths in e.g. a modem, or SDR ADC, to minimize harmful analog-domain interference at your bus frequency — i.e. to increase SNR, you're trying to make your traces be as little like an antenna as possible for the frequency bands they're carrying signal in, because you can't just band-pass that interference away.
The nice thing about shrinking by half, in such deigns — I would think — is that if you've already "tuned" your trace paths to a quiet band (for the country the component is being licensed in), then the harmonic frequencies of that band will also be quiet. Otherwise the band's fundamental frequency wouldn't be considered quiet!
(See also: why the unlicensed commercial-use spectrum was allocated to 2.4GHz, and then to 5GHz. 2.4GHz is an obvious choice, already useless for long-range communication due to water in the atmosphere; the other is its equally-useless first harmonic. But the great thing about choosing the first harmonic in particular, is that transmitting at 5GHz isn't putting short-range harmonic noise onto any lower bands that weren't already noisy due to existing commercial use of the fundamental frequency; so you won't suddenly find your other-band devices working worse in the presence of 5GHz transmitters than they already worked due to 2.4GHz transmitters.)
Not an antenna per se, but RF ICs often contain silicon inductors tuned for the operating frequency of the transceiver. Shrinking those down would retune the circuit for a higher frequency.
Even if you ignore all the other complications when switching nodes (and there are a LOT!), this alone prevents simple downscaling of circuits. It is very likely that after downscaling at least part of the interconnects have to be rerouted.
As for automation of that task: It's the traveling salesman problem in disguise. Which means that you CAN automate it, and there exists software for that purpose, but the result are hardly optimal, and most likely leave quite a lot of possible performance on the table.
Add to that all the other necessary changes when switching nodes, and it becomes fairly obvious that switching nodes, even if only porting 1:1, is a massive effort that can easily span YEARS.