The US has been reducing its troops in Afghanistan since 2011 and President Biden's decision to complete the full withdrawal by September 2021 merely extends by four months the original deadline established by his predecessor Donald Trump. US-funded private security contractors already outnumber uniformed troops by about ten to one. However, given the absence of material progress in peace negotiations between the fragile Afghanistan government – led by President Ghani and relatively more closely aligned with India – and the resurgent Taliban, relatively more closely aligned with Pakistan, means that the withdrawal of key US air and artillery assets likely alters the balance of power, or negotiating strength of these two parties, in favour of the Taliban.
US withdrawal suits Pakistan more than India...
In the short term, the desire for an orderly troop withdrawal will align US interests with those of Pakistan (to exert restraint on the Taliban) and the subsequent shift of the balance of power in favour of the Taliban suits Pakistan more than India.
...but Afghanistan's neighbours will be wary of insecurity spillover
In the medium-term though, Afghanistan likely remains in conflict, with the involvement of a range of competing international parties (China, India, Iran, Pakistan, Russia) each able to undermine whichever group dominates central government. Hopes of realising Afghanistan's potential as a mineral resources producer or gas transit route, let alone the ambitions of its 40mn population, are likely to remain unfulfilled.
The potential spillover of insecurity (or the vacuum of a strong Afghanistan government which allows non-state actors like Al Qaeda to flourish) is to no benefit of any of its neighbours, including Pakistan.