The problem with the extending the taxiway is the fact that it would have to be parallel to the runway (safeguarding issues). The problem with that is that Mitchell Way turns in towards the runway too. Therefore you have to straighten that out, and to do that do SOU own the land next to the railway station, would moving the road affect the railway track etc. You'd have to move the Glidepath to the other side...etc.. it all mounts up to a big job and a big cost.
Now could that money be better spent elsewhere? Somewhere, where you a guaranteed a decent return on your investment. To shareholders of companies need to know that the CEO/board is getting a good ROI, which in turn keep the share price up everyone happy. Except those on the ground who have to back track.