Lower standard of living is the answer, not immigration
A modest reduction in the standard of living and stopping the constant expansion in aspirational accumulation of 'stuff 'is the answer to the global westernized problem with Pension Provisions. The issue with funding US healthcare is another drain on resources where a UK/Canadian style State provisioned healthcare system is more economical than an insurance provided profit driven scheme.
Getting immigrant labor in to grow your economy to pay for stuff now just kicks the can down the street, as many pesky immigrants actually stay to reap the rewards of their labor and the Pension's shortfall snowballs in the future in 2-3 decades time.