I have gone #FullMonzo as of May 2018 but did it as a partial switch. The reason was that I wanted to get all DDs and salary to Monzo but keep the old account for things like cash deposits and international transfers into the UK.
If back then I did not have an option for a partial switch, I would not have gone to Monzo in full and simply transferred the salary as a SO (feel pity for those doing it, it is so much more convenient to just be with Monzo). Interestingly, as it has been said, I also had some of my DDs shown twice but that did not affect the spending (the right amounts were taken with no excesses) and they disappeared within a month or two.
Until Monzo allows for cash deposits and international transfers into Monzo directly, I believe many people will stick to using Monzo as a spending account first and foremost, simply because it is unable to facilitate all the services necessary. And now there are even less incentives for people to switch since the flexibility is removed.
To give an appropriate analogy, I have always agreed with card top ups removal from April 2018 simply because back in the days it was a necessity which Monzo could not be used without. The termination of that function was logical because Monzo has become a bank that could be topped up by a simple 1-second transfer.
However, forcing users (who may genuinely want to be with Monzo) to close their old accounts whilst not offering a full adequate alternative is not the best move though.