Fawcett's argument was concerned with the experiential structure of the verbal group: Finite and Event and the optional elements Auxiliary and Polarity. There was no discussion of the logical structure of the verbal group.
Now, as Halliday (1994: 196) pointed out, 'the experiential structure is extremely simple; and most of the semantic load is carried by the logical structure, including the tense system'.
The value of the logical structure in elegantly explaining the tense system of English was pointed out to Fawcett by Erich Steiner at the end of the presentation, despite the lack of time afforded by Fawcett for any discussion.
But consider also the larger theoretical picture. Removing the verbal group from the architecture of the theory does not simplify the model — it complicates it by creating inconsistency. Eg Whereas participants and circumstances are realised by units of the rank below, nominal groups, adverbial groups and prepositional phrases, processes are not. Somehow their realisation skips a rank and is realised by the verb. Try to explain a process such as 'will have been going to have been taking' as realised only at verb rank.
This all demonstrates that Fawcett neither understood the theory he was discussing, nor the theoretical ramifications of his suggested modification nor the principle of parsimony in theorising.