Amin,
You said "Actually, bcs one my relatives has Surface pro 3 he said that after Microsoft stopped supporting his device, step by step after a while its performance has been decreased and now he cannot use it since it crashes easily, and also he cannot sell it with a good price bcs his device is totally outdated. To wrap it up, Although my surface pro 7+ is doing my job, but I'm afraid of to having the same problem as my relative has.
From your view, Will I be able to use my Surface in the future, like 2029 or more? Or it will break and it will be much cheaper..."
No one can tell you for certain what to expect in 2029 and onward. The battery may have reached end of life, and Windows itself may have new features not supported by older devices. No one can predict if something else will "break". My own SP7+ has about 40% battery capacity left and I'm using it now as a test machine. The SP7+ launched in 2021 and won't be something that can be sold on the used market easily in 2029 and beyond.
Bottom line, this is a decision you need to make yourself as no one here can predict the future.