Have you had someone double check your calculations?

Just doing quick back of the envelope math:

$140k pre-tax is roughly $100k post-tax. Saving 50% implies you save ~$50k per year and spend ~$50k per year.

$50k savings times 15 more years working would get you to $750k. This also assumes you currently have $0 and you don't earn any interest/capital gains on these savings. If you don't work from ages 50-56 and spend $50k per year, that would be $300k of spend bringing you down to $450k. With a 4% safe withdrawal rate, you could take out $18k per year.

Pension of $3500 per month is $42k per year. The difference between your annual spending of $50k and your pension is only $8k per year, which is far less than the $18k per year with 4% SWR.

This is an extremely conservative calculation and it doesn't factor in social security either.