Not collapsed, not without Leningrad falling in 41 or something equally large, but they would have been hurting far far worse until the Western front in Europe is fully opened up.
That is an opinion i share. Without the western transport assets such as trucks and trains i feel the USSR would have suffered a LOT more casualties.
Not to the point where they might have lost the was, the massive distances and the German supply chaos were sufficient to avoid that, but in my opinion it could have delayed the soviet counter offensives by 6-12 months.
This by itself could have been beneficial for the western allies since the meat grinder in the east kept going no matter in which direction the front line was moving. This could have resulted in fewer forces in Italy and France to counter the invasions there.
Would the delay of no LL to the USSR have been sufficient to have an eastern front on Russian soil by the time the western allies crossed the Rhine or took Berlin THAT is something which is difficult to judge.
The inevitable cold war would be a lot more frosty than it was OTL though.
As for the reason for withholding LL, were there any assets from American or British companies confiscated during the Russian Civil war?
Perhaps a lack of compensation for those assets might lead to very strong anti soviet sentiments.
In the United States the isolationist movement was quite influential as were the Nazi sympathizers. Perhaps these groups can put a stop to the LL for the USSR.