Sales tax/VAT are all artificial constructs. If they don't apply to advertising (at least not in a per-ad-impression model), there's no reason they can't be tweaked to also not apply on a per-micropayment model, if accounting/collection for such tax ends up being too much overhead. That's the same reason why most tax regimes permit a tax-free allowance for individuals/sole traders under which there is no need to report income, since the overhead of taxing such little income wouldn't be worth the effort on any side.

There needs to be political will to do so, but if advertising does become unprofitable there will be pressure for such political will to exist.

Also, sales tax is mainly a US problem with every state rolling their own tax regime. Intra-EU VAT collection is actually pretty straightforward in comparison; unless someone claims to be not from EU or is an EU-based business, you charge the nominal VAT rate from your country, remit it to your government and call it a day.