One of the (basic) equations which models electromigration is Black's equation. It states that Mean Time to Failure reduces with cross-section of a wire and has strong dependence on temperature.
There are few factors which cause an increase in rates of electromigration (EM) when transistors become smaller and are placed more densely. The most significant are:
- The width of the metal interconnects decrease exponentially
- The total length of metal interconnects grows exponentially
The first factor leads to higher current densities (since the total current reduce over time, but not exponentially), and the second leads to an increase in space where a failure can occur (along the wires). Also both factors contribute to higher resistivity, thus increasing the amount of heat produced.
It seems like you might be right and EM failures are more likely today than they were 10 years ago. However, do not forget that design and manufacturing technologies are constantly evolving:
- Today's ICs are checked with special tools for EM issues at layout level (some tools even use predictive models to find EM issues after synthesis)
- Better materials are used (for example: copper instead of poly-silicon)
- Manufacturing processes are more EM aware
- Testing techniques and extrapolation models (for long time predictions) are better
In fact, in light of the "power reduction trend" which emerged after smartphones and tablets were introduced, I think EM reliability today can be even better than before. Anyway, no manufacturer will sell a device which is known to fail for EM issues before other HW fails (see comments to your question). Not unless this manufacturer wants to stay on market in the future.