Is a classic car a good buy? Many people think yes. But fundamentally its old technology, with all the trouble to find old parts, and less performance than something more modern.
Certainly there won't be any more made and Venom has some history to the "end to the spring era". Maybe there will always be some spring rifles made as they are pretty basic and great fun to shoot. Nostalgia is a great thing but eventually it will be PCP's that deliver that! Spring rifles will be for the historic collector.
Whatever invested in them then in truth they are great value. The fun of ownership and because they are just nice if not outright lovely. The return in ownership is the enjoyment from them, and you are likely to get your money back when its time to pass them on. Part of the hobby is cleaning and making sure they are kept in as good condition as possible.
Heck, other than tying up some capital and some elbow grease they don't cost anything.