SSGs are just a rerun of the 90's and don't solve anything that Varnish doesn't already. Pages are an increasingly quaint notion on an increasingly dynamic, personalised, multi-device web. Time to publish is ever more important as Google SEO measures news sources in minutes and publishing workflows often require instant publishing and internal search where there are handoffs (e.g. a subeditor and picture desk).
The two real problems I see is that ESI is our best solution to rendering (and is a clunky 90s-style design), and that object cache refresh has a tendency to combinatorial explosion.