Observation:
Expectation is that the number of bots will continue to increase and that the number of BotGroup will expand.

One way to help with maintainability is to split the bots.cfg file based on the logical associations, e.g. perf bots, skia bots, etc into relative files.

Proposal:

Add a new "import" to BotsCfg. This enables importing more bots configuration from another file:
  repeated string import = 3;

in
https://chromium.googlesource.com/infra/luci/luci-py.git/+/master/appengine/swarming/proto/bots.proto

Then update https://chromium.googlesource.com/infra/luci/luci-py.git/+/master/appengine/swarming/server/bot_groups_config.py to process these files too.

Then in bots.cfg, add at the top:

import: "bots_perf.cfg"
import: "bots_skia.cfg"