>Otherwise, what is to stop millions of poor masses from migrating to the country offering the best welfare and driving down the wages for locals (by, say, willing to work for a lower standard of living)?

Well, the math on that doesn't work if you're taxing all income rather than just labor income. Driving down wages while doing the same work will increase profits, so the same taxes ought to get paid.

And then there's the issue that adding labor to the economy should make it grow... unless for mysterious reasons like land-rents or parasitic finance systems all growth capacity is getting sucked away somehow.