V, 39 bytes

:im hello hello:se noma
:map : i
i

Try it online!

:im hello "Remap 'hello' in insert mode to
 hello "write hello, then hit escape
 :se noma "then set the buffer to not-modifiable
:map : i "THEN remap ':' to 'i' so that can't be changed
i "enter insert mode and await an infinite stream of input

Vim, 39 bytes

:im hello hello:se noma
:map : i
i

Try it online!

:im hello "Remap 'hello' in insert mode to
 hello "write hello, then hit escape
 :se noma "then set the buffer to not-modifiable
:map : i "THEN remap ':' to 'i' so that can't be changed
i "enter insert mode and await an infinite stream of input

Vim, 37 bytes

:im hello hello:se noma|map : i
i

This maps "hello" in insert mode to write hello, escape insert mode, set the buffer to nonmodifiable, and remap ':' to 'i' (now essentially a no-op) so that the buffer cannot be made modifiable again. Then enters insert mode to await an infinite stream of incoming (printable ascii + newline) characters.

V, 39 bytes

:im hello hello:se noma
:map : i
i

Try it online!

:im hello "Remap 'hello' in insert mode to
 hello "write hello, then hit escape
 :se noma "then set the buffer to not-modifiable
:map : i "THEN remap ':' to 'i' so that can't be changed
i "enter insert mode and await an infinite stream of input