If I remember correctly, the stripping of the atmosphere by solar wind is a very gradual process, such that if we did have the technology to terraform it on the timescale of human history (even a process requiring centuries to complete) the rate of human-effected change would far outpace the rate of loss. So a terraformed Mars would still be habitable for a long time, even if we didn't solve the magnetism issue.

Like, if you could magically give Mars a full Earth-like atmosphere today, it would still be breathable thousands of years from now (assuming solar wind was the only deleterious effect).