I fly on these birds regularly.
They are certainly workhorses, but the concerns described above are the exception, not the rule.
I personally haven’t had a technical problem on any of my commutes, and that numbers tens of flights already this year.
The 19 737-400s based at Gatwick were built between 1991-1996 and have (mostly) been maintained by BA fo their whole lives; this means there is a considerable number of years of serviceability left in these birds.
I find the seats comfortable, any new seats would likely not be convertible, or as comfy, so be careful what you wish for!
This fleet was on the radar for replacement, and the oldest 737s were indeed retired, but in my personal view they will not have any significant interior refurbishment until they are replaced as part of a larger shorthaul fleet order in concert with Iberia, and very much dependent on the success of the newly revitalised BA Gatwick operation.