You can keep the old weapons if you want, and I'm sure you can still use them for all the year-1 content. I'm all in favor for having the new influx of weapons, and making them be the effective choice for the new content is a good idea (in my opinion). Want to hold on to that <insert old weapon here>? Fine! Use it for everything before TTK, and use it in normal crucible, where level advantages are disabled! Otherwise, move on :)