in my opinion this is a different issue. In central London the zonal system does hold and fares are predictable.
I'd agree we (well, mainly me!) are putting two issues together here.
The original complaint relates to finding out what the fares actually are. I see no reason why TfL couldn't put up prominent posters at every station clearly stating the fares from that station to the other zones, plus cap information. But in many years of travelling the network, I don't recall ever seeing such a thing.
Being forced to use a PAYG system in the first place, as TfL effectively does, is a separate issue. Though it causes the other - as even if fares aren't advertised clearly, you'd at least be confronted with what they actually are before starting your journey, ie. when you buy the ticket.
I was talking to a friend today, who is uninterested in rail, who said precisely the opposite and that they know it could cost more but it just isn’t worth it to them and they still tap.
Which is fine - I still use paper travelcards even though on occasion they cost me more than using PAYG, simply because I don't want to use PAYG. But I can afford to do so, not everyone has the money to sacrifice.
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I don’t understand the continued references to a blank cheque; it isn’t difficult to discover what the fares are before travelling, or to check what you’ve been charged after the fact.
Even if you know the correct fare, you're still offering an entire pot of money to the company and expecting them to take the right amount. That's different from how every other transaction is done - you either hand over the correct amount of cash, or you approve a *specific* amount when paying by card.
TfL are worse still, because they effectively say 'the cost is £2 if you trust us to fill in your blank cheque, but if you want to fill it in yourself with the correct amount before handing it over, the cost is £5.' Nobody in their right mind 30 years ago would have accepted that, and I fail to see why we should accept it now.
If your card is registered you can view your entire journey history on the TfL website!
If you can be bothered to go through the whole process (which as I understand it also now requires a telephone for 2-factor authentication). I'd rather not have the hassle.
If you forget to tap in or out, or notice any other discrepancy (exceedingly rare), you can contact TfL directly who deal with issues very swiftly in my experience.
Which is fine if you're happy doing that. I want to offer the correct amount of money at the point of sale and avoid having to check/chase/quibble later.
What’s stopping you checking the fares online beforehand,
Why should I have to look online? Why can't the fares be prominently displayed at the stations?
and then loading a specific amount of money onto an Oyster to cover your intended travel,
...for now, but it's hardly a secret that TfL would like to get rid of Oyster. It will be rather harder to keep a specific amount of money in your current account to just cover the journey you're making (and rather dangerous, if you get charged for going overdrawn and TfL charge the wrong amount for whatever reasons).
or simply buying a travel card?
I do

Fortunately this is still an option, at least for now.
These methods are much less flexible and convenient than using PAYG via contactless, of course, which is why most people now use the latter.
Which is fine for those who choose to do so. I'm rather old-fashioned and prefer to be certain how much I'm paying up front, and I don't see why TfL shouldn't continue to allow me to do so.