The reason for reducing the limit on abortions is that we can now be fairly sure a 24-week feotus will survive and be healthy, which was not the case up until recently. It seems reasonable to lower the limit in the light of this fact, unless there is good reason for a later abortion e.g. catastrophic abnormality e.g. anencephaly. At the moment, as you say, very few women go past 24 weeks for an abortion and it is because of death of the child, conditions where life is insupportable, or where the mother's life is in severe danger. Such decisions would not be affected by lowering the limit to 22 weeks, because many of them occur now after 24 weeks.
Conflating abortion and viability is a dangerous thing to do.
For a start your numbers are wrong. The earliest baby to survive was 21 weeks and 1 day. I personally know a number of 23 weekers who have gone on to thrive with no issues.
The survival rate at birth for babies born before 24 weeks is 50%. But about half of those babies die within 6 weeks. 40% of those who survive go on to have lifelong / life limiting disabilities. A further 30% will have a less severe disability, but still will require long term care.
At 24 weeks, the survival rate rises, but is still only 65%. 20% of those do not make it home. Again, 50% of those who do go home, go on to have a life long disability, 20% of whom will be severely affected.
When people talk about "viable" they don't really know what they mean. Surviving birth is only a tiny part of the picture.
The reason we have to be careful about lowering the point at which a person can choose an abortion is down to pre-natal scanning and foetal development. It isn't until 20 weeks that many foetal abnormalities can be detected. If a scan is slightly delayed (which isn't unusual) you will put women in the position of having to make a very difficult decision in a very short space of time.
The system as it is, is working well. 0.1% of abortions are at 24 weeks. 1% happen after 20 weeks. 95% happen before 12 weeks. If we allow a political party to use abortion as a wedge issue, for political gain, we're all losing.