I've found that even the current minor rules aren't enforced. There should be someone monitoring renos and demolitions to make sure trees aren't cut illegally.

Agreed that it's ridiculously cheap to cut down a tree. Despite our lauded bylaws, bad stories still happen often. I'm not sure though that age is the single best way to determine what should be kept, though.
2 problems:

1. I'm trying hard not to anthropomorphize here. A vacant lot filled with tall, old, crowded and unhealthily spindly trees that were unwisely planted at the same time ought to be able to be thinned to a more healthy number without cost. Aesthetics/tree health ought to be a determinant somehow. Similarly, there are many old trees that have been amputated, not pruned, over the years, and in their resulting lack of heath they ought not to be valued over a younger tree that has had more sane treatment and is not suffering. It seems we haven't fully codified in law many features that most residents already can agree on, but maybe it can be done with some work?

2. A rising cost, especially on a logarithmic scale, while it may be appropriate by community value, also encourages removing trees now before they get older and more $$$ to remove. Need some offsetting factors to encourage retention of trees-- in rural Ontario for example there used to be a program that gave a reduction on property tax in proportion to trees kept on the land.

Trees are far more valuable to humanity than science is aware of. Why do have to cut any trees? I would prefer that we remove housing, fencing, lanes and sidewalks and turn the city into a forest with housing rather than a city with trees.

This should probably be a log or exponential scale, rather than a simple linear one, so that very old trees are considerably more expensive to remove - so a 100 year old tree would be $5000, rather than a mere $500, for example.