I have to disagree. An entry-level DSLR also has picture modes similar to a P&S camera, so you can get the shot if you're in a hurry and don't know exactly how to do it manually. I've had Pentax SLRs for 15 years now, and while the bodies get old, the lenses don't, as long as you look after them. I have a lens that's at least 30 years old but still works on the latest Pentax bodies. I'm never going to be a professional sports photographer or papparazzo - which are the only reasons I could need (or afford) the top Nikon or Canon bodies. For everything else, and especially for enjoying photography as an art form, I would not dream of switching away from Pentax now.
Why do you need the larger SLR lenses? Because they offer wider apertures (lower f-stops), and that gives you two things: better photos in low light (for a given sensor), and more control over depth of field (DOF). If you really want to get in to photography, you want to learn DOF control (which includes manual focusing skills): these days it is, in my opinion, the thing that can make a photo great (or not), and is a core creative tool to have. It is one way you make a photograph tell a story, making it about more than just the objects in the photograph.
Lastly: there's no shame in letting the camera help you at times - don't believe camera snobs who tell you're not a "real photographer" unless you go all manual, all the time. I make an analogy with driving: a stripped down car with a powerful engine and a manual stick shift makes for a rewarding experience on a racetrack ... but you still have to drive to and from the racetrack, and you don't want to be fighting your car when you need to be watching the traffic. With a camera, sometimes you want full creative control, and other times you want to be talking to the people you're shooting rather than futzing with your camera.
If you do use picture modes, though, learn what they do, and why, and that can be educational in itself. For example "Portrait mode" will open up the aperture to offer a shallow depth of field, so that the subject is in focus but the background is nice and soft. However, the camera can't tell you which lens or zoom level to use: portraits are usually better with longer lenses, in the 80-100mm (full frame equivalent) range, since that tends to flatten the features a little (not too much), flattering the subject. That's a guideline, not a rule, and anything goes creatively - but if you ever see a wide-angle close-up of your face, you'll understand why that's not really done except for comic effect ..!