TVs are a wildly unprofitable business. It's astoundingly bad. You get 4-6 months to make any profit on a new model before it gets discounted so heavily by retailers that you're taking a bath on each one sold. So every dollar in the BOM (bill of materials) has to be carefully considered, and not far back the CPUs in practically every TV was single core or dual core, and still under 1GHz. Bottom of the bin ARM cores you'd think twice to fit to a cheap tablet.
They sit within a custom app framework which was written before HTML5 was a standard. Or, hey want to write in an old version of .NET? Or Adobe Stagecraft, another name for Adobe Flash on TV?
Apps get dropped on TVs because the app developers don't want to support ancient frameworks. It's like asking them to still support IE10. You either hold back the evolution of the app, or you declare some generation of TV now obsolete. Some developers will freeze their app, put it in maintenance mode only and concentrate on the new one, but even then that maintenance requires some effort. And the backend developers want to shutdown the API endpoints that are getting 0.1% of the traffic but costing them time and money to keep. Yes, those older TVs are literally 0.1% or less of use even on a supported app.
After a decade in consumer electronics, working with some of the biggest brands in the world (my work was awarded an Emmy) I can confidently say that I never saw anyone doing what could be described as 'planned obsolescence'. The single biggest driver for a TV or other similar device being shit is cost, because >95% of customers want a cheap deal. Samsung, LG and Sony are competing with cheap white label brands where the customer doesn't care what they're buying. So the good brands have to keep their prices somewhere close to the cheap products in order to give the customers something to pick from. If a device contains cheap components, it was because someone said "If we shave $1 off here, it'll take $3 off the shelf price." I once encountered a situation where a retailer, who was buying cheap set-top boxes from China to stick a now defunct brandname on, argued to halve the size of an EEPROM. It saved them less than 5c on each box made.
For long life support of the OS and frameworks, aside from the fact that the CPU and RAM are poor, Samsung, LG and Sony don't make much money from the apps. It barely pays to run the app store itself, let alone maintain upgrades to the OS for an ever increasing, aging range of products.
And we as consumers have to take responsibility for the fact that we want to buy cheap, disposable electronics. We'll always look for the deal and buy it on sale. Given the choice of high quality and cheap, most people choose cheap. So they're hearing the message and delivering.