"The USA and the UK (etc) have no right to interfere in internal Afghani matters as such."

In what way are the USA and the UK interfering in Afghanistan's internal affairs? Do you deny that the "rulers" of Afghanistan came to an agreement with the United Nations (See Bonn Agreement 5th December 2001)? Do you deny the role of the United Nations in Afghanistan, the other 41 countries involved, the existence of UNAMA or ISAF?

"None of which addresses the OP's point - can the USA (and UK) get out without (a) getting massacred in the process or (b) leaving things worse than before or so that they will get worse than before?"

(a) - There are no massacres taking place after the same time in Afghanistan the Soviets had lost 15,031 killed. ISAF/US-OEF have lost just under 2,000 killed

(b) - Considering what was happening in Afghanistan before October 2001 under Taliban rule it is impossible for things to get worse, although the scale of the disaster if the participating UN member states cut and run as many advocate would be worse.