Why on earth don't they have adequate planning resources to replan a more appropriate timetable after almost three weeks when there was a perfectly good although heavily reduced (especially on the normally extensively served Victoria to Gatwick and Brighton line) service timetable that ran for 7 or so months in 2020 that they surely could have got up and running again within a few days in the week starting Tuesday 4th Jan and that timetable did not divert initially all and then subsequently still the vast majority of services in to London Bridge and also ran a shuttle service from Dorking to Horsham at off peak times including Saturday evening.
Firstly, there isn't the timetable planning resource to replan the entire Brighton Main Line in three weeks. In normal times, a timetable planning team is usually kept busy just with the usual alterations for engineering works each weekend - some weeks will obviously be more complicated than others, but it'll generally balance out. GTR's timetable planning team is presumably similar in size to what it was pre-covid - perhaps a little larger if they've managed to recruit, but it takes time to train up planners - and for most of the last two years all the people who
could be training new planners have been firefighting. The team over the last three weeks will have been busy planning for the blockade timetable for 19-27 February, or else beyond that if they're lucky enough to have got all that done in good time.
Secondly, the same timetable didn't run for 7 months or so in 2020. There was a quite substantial change at the May 2020 timetable change, as the Gatwick works started on schedule. That meant reworking the timetable and although it may have appeared to be largely the same, it wasn't. (Indeed, there were quite a lot of changes to 387 diagrams in particular - from May 2020 they started appearing on the East Coastway between Brighton and Hastings/Ore.)
Thirdly, in any case you couldn't resurrect that timetable today - because things have moved on in the meantime. The Gatwick works have moved on to a different phase, with Platform 7 having reopened and Platform 5 having closed. Thameslink paths on the Midland Main Line changed significantly in May 2021 as part of the East Midlands Railway timetable recast, so there's no possibility of going back to anything from pre-May 2021 without changing EMR trains (which simply wouldn't fit with the old timetable). You can't go back to an old timetable any more than you can replan the whole thing on the spot. And piecing together a timetable - say by putting current Thameslink paths together with the May 2020 reduced Southern timetable - would take just as much work in validating the timetable before it can run.
I think a lot of the replies in this thread miss the main point. It was been predicted many months ago that Covid was likely to flare up again in the winter, so why did GTR not have a plan in place for what to do if they had significant staff shortages? There should have been no need to do "replanning" at short notice if they had had a bit more foresight.
Because that, too, would take timetable planning resource that doesn't exist. There was no time to create contingency plans any more than there was to create a new base timetable.
Furthermore, it appears that the omicron wave has affected the South London Metro driver depots more significantly than the driver depots further south - after all, the overall frequencies south of Croydon are largely similar to normal (admittedly with discrepancies like Dorking-Horsham), whereas running such a limited Metro service must mean they're saving a huge number of Metro drivers at the minute. Could you have predicted that a plan would have been needed for significant staff shortages in one part of the network but not others? If they did have an off-the-shelf plan to deal with staff shortages, it was probably predicated on a level of absence that was fairly uniform across all depots - which would not have been terribly useful in the present scenario.
Let me be clear: I don't like the present timetable. It's an awful, horrendous fudge. But I genuinely believe it's the best that could be done at short notice. By the end of February, the blockade will be done and hopefully we'll be back to something closer to normality.