I was once conned in the railway whereby I leant on a standalone reader at Kings X without even realising! Wallet in back pocket, beep, TFL mugged me of £5 or so.
Since then I'm 100% contactless-free ! They can keep the dam thing...
Ironically if you'd had two or more contactless cards in that wallet then you would have experienced 'card clash' and not been charged!
I get around the scenario you experienced with ease. I bought from eBay some RFID blocking sleeves which prevent contactless cards from being used - unless the cards are removed from the sleeve. I tested them with an Oystercard on a ticket machine (as I was not planning to travel at that time) and happily I verified that the sleeves work.
I have never used contactless to pay transport fares but am also astonished that simple card reads on transport access terminals which do not actually invoke a payment at that moment are causing such woes.
What if the OP's card had been accepted for the 521 bus fare but was then declined during a ticket inspection? These inspections only read card details which are then reconciled behind the scenes. But if the read is declined for the ticket inspector would he / she then accuse the passenger of presenting the wrong card? Or have I just unwittingly identified a new way for ticketless travel, specially on buses and at ungated stations?
Other scenarios that would also be unacceptable would be the inability to use a pink route validator during a journey or to 'touch out' at the end of a journey (so that a maximum fare is charged).
Happily Oyster cards are not likely to be phased out for a long time, as there will always be passengers who cannot have bank cards (under 18, bankrupt, short stay overseas visitors, etc). However I understand that at some stage within the next few years Oyster cards without a letter D in a blue box on their backs will stop working. This is because these cards use older less secure data transmission technologies which will be switched off at the card readers. However I would expect passengers to be able to swap their older Oystercards for new Oystercards, with their e-purse values and any stored season or discount tokens (eg: railcard) also being transferred. What I do not know is whether the swap will require the new cards to be purchased with the old cards having the deposit refunded.
As for weekly fares capping, its my view that in many cases passengers will be better off buying weekly seasons, if they are still available and cost the same amount. This way all complications (such as journey time limits and forgetting to touch in / out at stations with open gates) are avoided.
Simon