The chances are that we will have to fight Russia
True, but the longer Russia fights Ukraine, the weaker it will be at this point and we will have more time to prepare.
the weapons we sent will be used against us.
That would only be a problem if Ukraine suddenly collapses. The Ukrainians have so far put our weapons to good use and besides, even if the Russian capture them, they lack the knowledge to maintain them.
Furthermore the Ukrainian war has given us valuable feedback on how to improve them.
But we need those damn
There is a good that we won't need them if we continue to support Ukraine.
be arsed conducting a proper mobilisation
Do you think the Ukrainians are not properly fighting the Russians after 2 almost two years of war and likely more than 100k casualties?
You have probably read the reasoning behind the decision. The Ukrainians can only train so many new recruits at a time, and they can not afford to sacrifice them like the Russians do because they have less people and because doing so would weaken domestic support. Not to mention that they also need a modicum of economic activities to keep the country afloat.