They still mentioned them and, maybe your above that, but it created expectations.
You're right of course - though from my own POV, said expectations were "conversion docs will appear in early 2015", and with the new info re: jury duty my expectations are now "conversion docs will appear a few months from now." I
do think it would have been better to not mention it in the first place, but when expectations need to be adjusted (either due to changes in circumstances, or said expectations having been faulty the entire time), they need to be adjusted, and I'm not sure I see how Mearls' tweet failed to do that.
So what to conclude?
#1) The best policy is to have a schedule, make it public, stick to it as close to possible, but be open and candid about difficulties.
#2) The second best policy is to have a schedule, but refuse to hardly ever talk about it or make it public
#3) The worst policy is to have a schedule, make it public but refuse to explain why you are not keeping it
I'm not sure I completely agree with #2 being much better than #3 as that means your company drops off the radar somewhat as people aren't anticipating anything from you. But even by this analysis, WotC is pursuing a suboptimal policy of disclosure at the moment.
In general, I'd agree with your ranking. However, in this particular situation, what I'm concluding is that #1 is optimal
if said difficulties are primarily out-of-house - but what if your primary obstacle is "supporting the tabletop line is of secondary concern to WotC, as our primary focus is on the larger brand, i.e. board games, films and video games based on the property"?
In that case, I'd say sticking with #2
is optimal. Flat out saying such will not gain them anything and can only hurt them.
Not saying anything at all might also end up hurting them in the long term, but if they're looking to wean people off of the prodigious product schedule D&D has been synonymous with for the past few editions I can't see a better way to do so than they're already doing.