That's not necessarily true. If you buy a Travelcard from stations outside London, the additional cost over a London Terminals ticket can be significantly lower than the daily cap on Oyster and Contactless. Also, with an Offpeak Travelcard, you don't get charged peak fares between 4pm and 7pm on weekdays. The discount depends on the TOC and the route, but it's worth checking what's offered.
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The more I think about it, the more I consider that those irrelevant and expensive ticket offers on the SWT website are close to fraudulent. They are certainly very comparable to the PPI mis-selling that the banks were forced to pro-actively give refunds (plus compensation!) for. It would be interesting to hear from staff how often they come across day returns like these (at weekends and on bank holidays) that charge large extra sums for absolutely zero extra benefit to the passenger.