The best replacement for mainframe is server.

The best alternatives to mainframe are rated as following:
★★★★★  server
★★★★  client server
★★★  pc
★★★  distributed
★★  unix
★★  midrange
★  supercomputer
★  distributed platforms
★  open systems
★  server farm
★  as400
★  microcomputer
  personal computer
  open system
  distributed environment
  distributed systems
  midrange computer
  client server architectures
  iseries
  client/server computing
  midrange computers

Your alternative for mainframe is not listed? Suggest!
Speaking about mainframe, one probably shouldn't forget about teradata.

Other search terms related to mainframe and their ratings are:
★★★★★ teradata
★★★★ sas
You have in mind a companion for mainframe and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mainframe in all years. Click on graph for more.
mainframe
server
client server