ElectricMonk either you don't know what supply side economics are or you don't care for starters there is a lot more to it than the Laffer curve, including the very important work that Reagan did lowering barriers to entry and cutting red-tape, Laffer=/=the totality of Supply-side economics. As to the Laffer curve the debate isn't whether it exists, everyone to the right of Karl Marx accepts that a 50% tax rate raises more than a 100% tax rate in the long run, the question is where the top of the tax curve is. In the case of the UK the Thatcher tax cuts undoubtedly raised revenues, in the case of the US which started off with lower rates the evidence is less clear.
As to the OP no Reaganomics and a continuation of 1960's style Keynesian demand management results in a much poorer US with high structural inflation, a debased dollar and internationally a much weaker China and a comparatively stronger Europe, though everyone would be poorer. As for Reaganomics with a dash of Keating style budget discipline then a much better world, and a richer US in the long run. It would be poorer than OTL in 1990 without that debt fuelled boom but richer by 2010 without the debt burden slowing growth.