Hi Linda, and welcome! I'll try to give a summary answer, and I expect many others here will jump in as well.
Firecalc tries to show what could happen, and help you figure out how to reduce the chance your money will run out before you die.
Because of this focus, it's a very pessimistic tool, and if the economy in your remaining lifespan is anything better than the worst that we have ever seen, you will have a tidy sum in your estate.
But that doesn't mean you have to follow the plan for the rest of your life -- you can readjust whenever it suits you.
Let me give you an example.
Suppose you are planning to retire and you feel it is prudent to make sure you are financially prepared for the next 40 years. Using this tool or others like it, you determine that you can spend $40,000 a year, with adjustments for inflation, and still make it for 40 years.
After a few down years in the market, you are still living your planned lifestyle, because your spending ability has remained intact.
Then you get a few boom years in a row, and your portfolio really grows. At that time, why not recalculate? Using the increased portfolio, and the reduced number of years it has to last, you might find that $60,000 will still be safe.
The interesting thing here is that you get to maintain your lifestyle following bad years, and improve it following good years.
That sounds like "something for nothing", but all it is doing is reducing your likely estate.
There are other ideas bandied about here as well, so I encourage you to read more, ask questions, and enjoy your early retirement!
Dory36