..... The agreement was never off the table or renegotiated. In fact former President Trump even aluded to this fact in a statement on June 26, 2021. During the middle of the final withdrawal.
June 26, 2021 — At a rally in Ohio, his first since leaving office, Trump boasts that Biden can’t stop the process he started to remove troops from Afghanistan, and acknowledges the Afghan government won’t last once U.S. troops leave.
“I started the process,” Trump says. “All the troops are coming back home. They [the Biden administration] couldn’t stop the process. 21 years is enough. Don’t we think? 21 years. They couldn’t stop the process. They wanted to, but it was very tough to stop the process when other things… It’s a shame. 21 years, by a government that wouldn’t last. The only way they last is if we’re there. What are we going to say? We’ll stay for another 21 years, then we’ll stay for another 50. The whole thing is ridiculous. … We’re bringing troops back home from Afghanistan.”
So yeah, even Trump through the summer of 2021 was still boasting about his deal with the Taliban, and how Biden couldn't stop it. He wanted all the credit, but then .... not surprisingly wanted to quickly erase that history 6 weeks later after Americans lost their shit because we were not given a hero's send-off in August.
Also, just a reminder that by January 15, 2021 we only had 2,500 remaining troops left in Afghanistan. That wasn't going to be enough to do much of anything from a strategic perspective. So if the US wanted strategic goals accomplished (eg retaining bases, withdrawing equipment, etc) it should have done that prior to this point)
Jan. 15, 2021 — “Today, U.S. force levels in Afghanistan have reached 2,500,” Miller, the acting defense secretary, says in a statement. “[T]his drawdown brings U.S. forces in the country to their lowest levels since 2001.”
After January 15th, 2021, the US at that point was extremely reliant on the 300,000 Aghan security force -- which rolled over in the early summer of 2021. It's that last fact that made the withdrawal chaotic. But the US was too far committed to the withdrawal at that point. Americans (including the Biden administration) were never going to support a troop level increase in 2021.
The only way out was to see the withdrawal through to the end.