The parliamentary session running from June 21, 2017, to September 9, 2019, was the longest on record. It ran for 810 calendar days. To allow it to continue would have defied precedent and pushed parliament further into unchartered waters. 
If, as Johnson's critics claim, convention is essential to the functioning of government, then they should celebrate the prorogation, not criticise it.
Secondly, the length of the prorogation was also not unprecedented. Much of the period between sessions was already covered by the three-week parliamentary recess already planned for the party conferences. If you take this into account, the actual length of the prorogation was only seven days, not 33. This is a perfectly normal period of time to have between parliamentary sessions.