It always intrigues me how people get so obsessed with formations. In reality there isn't too much difference in many of them. For instance our standard 4-2-3-1 last season is hardly any different to the sudden flavour of the month 3-4-2-1. Rose and Walker play as wide midfielders as opposed to full backs, Toby and Jan fan out to play wider than a usual centre back pairing, Dier drops in between them playing a role similar to the old 'libero' type role. Eriksen would typically play narrow to make a central midfield 2 with Dembele and Lamela would join Ali in the pockets of space in central areas in the last third of the pitch.
I don't entirely agree, and I think the change to a back 3 is pretty vital in combating a 4-2-3-1, and I think the teams that have played a back 3 against us (Everton this season, West Ham away last) have shown that it can stifle us pretty well, and we play pretty much the archetype 4-2-3-1 with narrow creatives behind the striker.
With a 4-2-3-1, we have the creative guys cutting inside, all looking to thread through a central striker, who is isolated. Not only that, but if Dier/Wanyama drop to collect the ball, a team that presses us can quite easily isolate our holder. It means we can't get the ball up the pitch and inevitably hoof it, causing what Poch described as the ball not arriving in the final third in a good condition.
3 at the back doesn't make too much difference to our full backs as they play high anyway, but it means if teams press us playing their 4-2-3-1, we have more options to play around that. Then getting into the final third, we'v'e exploited the space and get closer support to our striker. Effectively a 4-2-3-1 wouldn't be that much different to a 4-4-2, in that sense you are right, except we are playing with creative ball players high up as opposed to wingers supporting two strikers. If we played wingers, we might well be suited to playing against 3 at the back because we are exploiting the spaces.
As Wilson said, it's all about exploiting the spaces, and I'm just glad our manager is one of the ones that is capable of being highly adaptive. Tactics are usually cyclical as one system generally gets an advantage over another until teams adapt (Mourinho's original Chelsea 4-3-3 giving an advantage over the standard 4-4-2s at the time being one other example). Chelsea's current 3-4-3 exploits most teams playing a 4-2-3-1, and a 4-2-3-1 and a 4-2-3-1 was likely a response to teams adopting the 4-3-3. The interesting thing is rather than cycles taking 5 or so years, we may see a few within the same season as there''s a lot of good coaches trying to eke advantage.