Monzo launches tool that allows customers to undo payments

Misses the point. 
A2A wins, imho, when the transmission entity has contractual, rules based, authority with all connected parties to unwind a payment days later. MC/Visa retain grip partially because the Chargeback system is robustly managed by them. 
I am not well versed on the mechanics of A2A but is it not possible to replicate CB functionality within the A2A flow?

TheA2A is under-pinned by the payment services legislation with its key principle on irrevocable credit transfer.  I.e. if you want ypur transfer back, legally the payee needs to send a new A2A to you. Banks cannon charge it back. This rule is the back-bone pf credit transfers and overlooked by consumer payers as a major weakness in customer protection and much in favour of e-commerce merchants and other payees that want to secure the funds. If a bank wants to create a "cancle time-frame" it needs to be agreed with the payer that all approved A2A payments will be delayed by x minutes when the payer can cancel the payment order. So an approved payment order would not be processed immediately by the payer bank but only with an x minute delay during which the payer van stop the payment.  If this kind of a delay is to be used for real time payments, it must be made clear to both payer and payee that the payment is approved for processing only after the x minute delay.  In general the A2A is giving the payer much weaker protections in merchant payments compared to card payments which have built-in customer protections for "services not received" or "wrongful delivery" situations. The A2A does not have these protections since the service has been built for transfer to another customer´s account and bill payments and not for situations where one buys something from an e-comm site and thus pays in advcance. many e-comm merchants would love to receive more over A2A in instant paymenmt since to gives them an absolute grip on the transferred funds. Nobody in the payment system can take the monies back. This is clearly to the detriment pof the payer. 

@Shane OHara Engineer: Actually no, you're missing the point! As I wrote in Why Don’t UPI / Zelle Provide Fraud Protection?, credit card solves for payor whereas A2A solves for merchant, never the twain shalt meet!! IOW, revocability / chargeback is a feature in credit card but bug in A2A. 

TIL sending a wrong amount is such a common issue in A2A payments like FPS. It can be minimized - if not eliminated altogether - if only the payment screen displayed the amount in words as soon as the payor enters it (in figures). Seems like such a simple feature but I've come across it in the netbanking of only one bank out of 10+ banks I've banked with.

This is a bizarre feature - perhaps reflective of the unthinking society we live in today. Instead of checking, double checking, then paying, it is a case of paying, then thinking about it, then checking.

Monzo is encouraging carelessness.

For those who are diligent, it means adding an artificial delay into a real-time payment, somewhat negating the benefits and adding friction.

Wild guess: Any payor who uses this feature or opts out of using this feature - basically every payor - and loses money by either paying the wrong payee or the wrong amount will automatically forfeit their right to compensation per PSR APP Scam Reimbursement rule. Banks shouldn't have allowed this Drunk Under Lamp Post rule to have come into existence in the first place. Having failed there, they will try to scuttle it in any way possible as the next best option. Monzo is just the first. 

@Ketharaman Swaminathan
The chargeback process for credit and debit cards does not solve for the  payor and not the merchant as implied. It is a two party process that aims to resolve disputes arising from a money transfer. Having sat on the issuer and acquirer side I know that all Issuers have to sumbit thousands of chargebacks which were not even requested by the Payor. Over the years. fraudulent merchant setups, bin spamming and other techniques used to empty consumers accounts. The Issuer chargesback and if the retailer is right and it goes beyond second chatgeback to arbitration, the win will be to the retailer. If the retailer is in the right, and the schemes have well established rules to adjudicate, the money reverts to the retailer. Whilst it is a painful process at times, it is a balanced process. The A2A process, on the otherhand simply has no process for addressing a bad transaction. So I am not sure what point I missed?

Oh c'mon, retailer can at best "not lose" in a chargeback, there's no way a retailer can "gain" from it. Whereas a payor can "gain" or at worst "not gain" from chargeback. No amount of hoops to jump for completing the chargeback process can change that basic point. If you want another fundamental point, it's that credit card is designed to a revocable MOP whether an A2A is designed to be irrevocable MOP. Therefore, at the highest level, it's self-evident that credit card is pro-Payor and A2A is pro-Merchant. 

Now that was only at scheme-level. 

There's nothing stopping other nodes on the A2A network from offering some kind of recourse for wrong payment. And some of them do. In the case of A2A UPI in India, there are nearly 50 PSPs like Walmart PhonePe, PayTM, CREDpay, et al, some of them do offer some sorta chargeback for bad UPI transactions.