Imo you should be beating isp on avg.
But you've raised an interesting question do those that have drifted to say evens, stayed at evens or shortened to evens all have the same chance in reality? I.e. if you looked at the long term strike rates of those 3 scenarios what would you see?
There's a very nice chap who I've known since the 1990s who works in the betting industry in marketing and, unlike a lot of such people, he has a genuine knowledge of racing and betting and could have just as easily become a racing journalist.
He's very interested in this sort of thing and once told me he'd seen charts of records dating back to WWII which illustrate how efficient the betting market is when it comes to the industry SP.
3/1 shots since WWII do that bit better than 100/30 shots, 9/2 shots do that bit worse than 4/1 shots etc.
If you blindly bet any horse at any specific SP you'd be behind - no reason to suspect it's any different for the BSP.
IIRC I asked him the same question about the journey to the SP, drift, shorten, or steady - he said it was a good question and he hadn't seen figures for that, but would like to!
The shrewdest and most successful punter I ever met (and I've met a few such people) ran a data model on Betfair and he too had huge respect for market efficiency - if his model (based on about 90 factors taken from 15,000 co efficients he'd examined) threw up a bet he'd have more on it the shorter price it was when he got involved (about a minute before the Off due to his volume).
It contradicted the Classic, purist, "the bigger the price, the bigger the bet" mantra of some older punters - I personally think there's a time and a place for both as the market doesn't always know, especially in niche markets, the market thinks it knows, but sometimes it really and clearly doesn't.
Anyway, this guy steadily ground out profits on a tiny ROI and said, if he started with a grand bank on 1st January, if he never skimmed the account to live on, he'd expect it to be a million by 31st December.
And everything I saw in my time with him validated it - I learned more off that guy in a year than I did in 20 at places like the Racing Post, or even working for bookmakers.