It's almost not harder at all to say, especially if the next consonant is velar (i.e. /g/ or /k/). The reason people say /ɪn/ as opposed to /ɪŋ/ or /iŋ/ at all is because the two morphemes -ende and -inge as a verb suffix essentially merged but some people favoured one pronunciation over the other, but it didn't ever hinder meaning.
To this day different dialects use different variants as a result and some even use both in free variation (like me). It's hardly "easier to say" as much as it just comes more naturally, because we've heard it and used it growing up. I doubt it would be heard so much if the only pronunciation were /ɪŋ/, if at all. I'm not ruling out the possibility of it becoming /ɪn/ an as eventual development, though.
There is more to read here, if you like: http://en.wikipedia.org/wiki/Phonological_history_of_English_consonants#Phonological_history_of_ng
As Barrie said, the orthography is very very unlikely to follow.