(note: this answer concerns the general notion of allowing a political party to assist in protecting its candidates, not the desirability of switching approaches at the last minute before conventions in the coming weeks).
One interesting question is whether parties would be willing for the really high costs likely involved in hiring, vetting, training and moving around a professional security detail (hiring some cheap locals may very well end up causing more problems than it would solve).
Currently, the Secret Service is paid for by federal funds and local law enforcement costs tend to fall onto... local taxpayers.
Presidential Campaign Security: What to Expect When a Candidate Comes to Campus - Campus Safety Magazine
The campaign staff will always tell you it is the Secret Service’s responsibility to pay police officers since it is part of the protection of the candidate. However, do not expect to be reimbursed by the Secret Service; they do not pay for your service.
Certain elements of the crowd management plan could very well be considered part of the protective operations. Is traffic direction part of the protective operation? Your institution should plan on covering the cost of additional police officers and other security staff. One consideration would be to build some of the cost into the pric e of the venue.
Note: I XXXed the name below because it really doesn't matter overmuch - I suspect the non-incumbent POTUS candidate behaves the same way in most cases.
Why the XXX campaign won’t pay police bills – Center for Public Integrity
But when Lebanon City Hall sent XXX’s campaign a $16,191 invoice for police and other public safety costs associated with his event, XXX didn’t respond. XXX’s campaign likewise ignored Lebanon officials’ follow-up reminders to cover the sum — one rich enough to fund the entire police force for nearly two days in this modest city of 21,000, between Dayton and Cincinnati.
But in many of these cases, there are no signed contracts between the municipal governments and the XXX campaign. The cities dispatched police officers to secure XXX’s events because they believe public safety required it — and the U.S. Secret Service asked for it.
Now, $20000-ish doesn't sound like that much. But it's only for existing local police overtime for one day. Deploying a full protective detail that would move the needle would involve hiring, vetting, paying full time salaries (of people who would have be highly qualified to matter) and all the logistics of moving a big team around. I could easily see that reaching in the hundreds of thousands for each day's events. Are campaigns willing to reach in their own pockets to that extent, rather than spending their cash on ads and other direct campaign spending?
Well-done (and, yes, coordinated with the Secret Service) this could be a big improvement on the under-resourced specialized forces present in Pennsylvania.
But the Post reported that it had seen an exchange of messages in which a former Secret Service officer asked colleagues how the suspect had got a gun so close to Trump. He was reported to have received the reply: "Resources."
I saw a POTUS security detail in Paris in the mid 90s. Flying The Beast in is a very different look (and somewhat patronizing to the host nation) than not being able to secure or overwatch a rooftop 130m away from Trump's podium. In that sense, it's hard to see how a properly sourced campaign security team would be more blowback-prone than current arrangements.
(nothing against the local cops btw - it's just not their day to day livelihood).