I think you are right. Everywhere I trim mine, to keep it back from the lawn, stops blooming completely. Now, only the very top that never gets trimmed is the only place I get flowers on my Persian Lilac. It appears that blooms only come from new growth on old wood. I am wondering if a severe trim and then just leaving it to grow untrimmed for several years would rejuvenate it. Over the years mine has outgrown it's allotted space. I had no idea that they grew so large.
I prune the oldest third from my lilacs every year and they bloom just fine. I cut out the thickest and oldest parts.
My nextdoor neighbor’s lilacs have never been pruned and they don’t bloom well at all. Hers are about 15 feet tall.
We have the same variety and neither of us fertilizes them.
I was told not to trim the old flowers off lilacs, because I might cut off next year’s flower buds which had formed over the summer.
Any parts you cut back hard won’t flower for at least two years, if then.
OTOH....
I’m in eastern Ontario, and I have noticed my Lilac is bereft of flowers, except on the top this year. Maybe they need a colder winter?