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"There are certain H categories where there are simply no qualified Americans. Revoking those will bring those industries to a stall."

Whether it is an electrical power plant or nursing or whatever, there are qualified Americans. The supply and demand may not be in the employer's favor in certain fields, and it might cost them more $$, but there are at least closely-qualified Americans. It is possible that a little cross-training or job specific training might be needed, but that was once very common place.
With the proper incentives, like $$, college classes for every needed job skill will fill with Americans. And, personally, I don't care if half these students need to start with remedial math; as long as they are Americans, we should be investing in them, not foreigners. Our children may lack the motivation of many non-citizens when it comes to doing their homework, but their I.Q.s are more than sufficient.
H & L visa workers remove the incentives in every field they are hired into, leaving an unhealthy void in the American workforce. Thereafter, the H & L visa workers will be indespensible.