The best replacement for 100-400 is 400.

The best alternatives to 100-400 are rated as following:
★★★★★  400
★★★★★  70-200
★★★★  sigma 150-500
★★★  70-300
★★★  400 5.6
★★★  70-200 with extender
★★★  150-600
★★  sigma 50-500
★  70-200 canon
★  canon 400 f5.6
★  canon 400
★  nikon 80-400
★  28-300
★  70-200 2x
★  sigma 120-400
  300 f4
  400mm
  70-300l
  50-500
  120-400

Your alternative for 100-400 is not listed? Suggest!
Speaking about 100-400, one probably shouldn't forget about 2x extender.

Other search terms related to 100-400 and their ratings are:
★★★★★ 2x extender
★★★★ extender
★★ 7d
You have in mind a companion for 100-400 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of 100-400 in all years. Click on graph for more.
100-400
400
70-200