It's true. Once bitcoin has extensive side chain functionality, altcoins will be essentially phased out, unless they convert into a sidechain.
Altcoins will not be phased out like what this user said:
Altcoins will not cease to exist but continue to co-exist with whatever Bitcoin is bringing to the table.
There will still be someone left to use those altcoins with functions so long as it is usable, depending on the situation some users will always tend to use some alts because it is more efficient to use in their current state(great proof why it will happen is the way how people still use candles and any other old lighting devices despite having light bulbs and flashlight today).