The Pentagon knows it can not exit Afghanistan on its own terms, so it would rather just keep it forever
The surge worked! That was the buzzphrase of 2008. Supposedly the 20,000 extra troops Bush ordered into Iraq for a year had calmed the situation so much the US could now proclaim victory and gradually withdraw from the country.
In reality the extra troops had done nothing. What had really happened was that the US – as it had done in so many of its wars – finally accepted a deal it could have had all along. US accepted local insurgent control of Sunni areas, and these in turn booted out foreign fighters and al-Qaeda in Iraq.
Nonetheless the surge myth was born. All the US now needed to do, to likewise win in Afghanistan, was to "surge" there as well. Obama thus allowed the Pentagon to arrange a similar escalation in Afghanistan – under the understanding that there would be real progress and that, like Bush, after a year he could gradually withdrawing.
Of course that was never going to happen, because the troop build up was never the magic ingredient in the Iraq "success" in the first place. Pentagon always resented the fact that Obama in return for okaying an escalation demanded to see some actual progress. For Trump they have an entirely new scheme: they want a massive new investment without having to show anything for it:
Most important, the strategy would jettison President Barack Obama's approach of setting arbitrary deadlines for the withdrawal of U.S. forces and instead would link the participation of U.S. troops inside the country to meeting clear conditions on the battlefield, such as winning back territory from the Taliban and denying safe haven to al Qaeda, the Islamic State and other bad actors, according to these officials. Pentagon wants a new surge into Afghanistan, but this time they don't even want to promise there will be any visible progress any time soon. Of course, the generals are right that a time-limited surge was never going to work. (Though they claimed just the opposite in 2008.) With occupiers fighting so they can leave on their own terms all the local resistance has to do to win is wait them out. But what is the alternative? Keeping the place forever?
According to the Pentagon and McMaster, yes that's a valid alternative. By jettisoning any "arbitrary" deadlines the Pentagon is saying just that – they're willing to stay forever "if necessary".
That however just means they have no clue as to how to "win" the war in any reasonable or unreasonable timeframe, at which point why continue fighting it at all?
At this point the US is on course to suffer an even bigger fiasco in Afghanistan than did the Soviet Union. The Soviet project to have a friendly and stable government in Kabul crumbled into dust in 1992 but at that point the USSR was no longer around to see it.
Unlike the Americans, the Soviets were actually able to withdraw from Afghanistan in 1989 and leave behind a somewhat sturdy pro-Soviet government. One that it turned out was sturdier than the Communist government in Moscow and went to outlive it by a full year, crumbling only after it had been left to fend completely for itself even as Pakistan and the US ramped up aid to its enemies. At this point even such meager "accomplishment" as the Soviets had in Afghanistan looks beyond the scope of the Americans.