I'm not so sure. Michael Bates was fairly well known back then and was in It Ain't Half Hot Mum at the same time as LOTSW. He did a couple of years of IAHHM after leaving LOTSW and it was very popular at the time, so he was a recognised face on tv.
The question is if Bates had continued, had he lived on and done say 9 or 10 series and then left, who would have come in at that point? Who knows, but Roy Clarke might simply have decided to stop writing it at that point and do something else.
LOTSW was certainly different in those first two years, the characters were more earthy and gritty. At that time it wasn't a gentle comedy, but wasn't too hard either. I quite like the character of Blamire, but he has to be looked at in the context of the time. That first trio certainly had their adventures, so in that respect it was no different from what was to come later. The characters probably would have changed slowly over the years had Blamire still been in it, just as they did after Foggy came in. I think it would have maintained its success as a series, but the question of transition to another third man might have been less likely had Michael Bates been in it for 10 years rather than 2.