The Trust has recently faced criticism from various corners, including a reported pledge by the Conservative Party to axe the body if it secures power at the general election.
However, Trust chairman Lyons told the Manchester Statistical Society yesterday that he views the organisation's primary mission as being to halt the "imperial compulsion" within the BBC.
Lyons claimed that BBC director general Mark Thompson's strategy review, which recently called for the closure of digital stations 6 Music and the Asian Network among other recommendations, will result in a "more compact" BBC.
However, he stressed that the corporation would not be "put in a straitjacket and never again allowed to do anything new" following the changes.
"The BBC needs to concentrate on its important and widely valued public role rather than seeking to become an international communications company," he said
Lyons claimed that many of the Trust's achievements have been overlooked in the flood of criticism, while the organisation's relationship with Thompson has been beneficial rather than a hindrance, as former BBC director general Greg Dyke recently suggested.
"Being part of the BBC keeps us close to the coalface [and] I believe strongly that having the Trust as part of the BBC is a strength not a weakness," he said.
"When we see things going wrong we can act quickly and decisively to put this right. The public would be short changed if the Trust were replaced by a glorified complaints office."
Lyons also denied claims from culture secretary Ben Bradshaw that the Trust is conflicted as "both regulator and cheerleader" for the corporation.
"What the Trust is not is the BBC's regulator," he said. "That's the job of Ofcom. Our job is to steer that tricky course between independence and accountability."
The Tories reportedly want to scrap the Trust in favour of a "licence fee payers trust", with a new non-executive chairman appointed to support Thompson in times of crisis.
However, Lyons said that there is a "kind of imperial compulsion" in the BBC's DNA which requires "clear boundaries" to be set. He believes that the Trust is well positioned to rein in the BBC's expansionist tendencies and avoid any negative impact on the choice of consumers.
"According to this view, the BBC is driven by an insatiable desire to expand, to colonise, to establish its forces in every far-flung corner of broadcasting and publishing. That is not something the Trust, as the representative of the public, will allow to happen," he said.
"You, the public, would be the losers. The Trust is clear that the BBC must be a good corporate neighbour to others in the media marketplace."
He added: "We have no issue at all with the BBC competing ferociously where it matters, on quality, but the Trust has no wish to see the BBC reassume its monopoly position."
Despite admitting that the BBC can seem a "pretty big and insensitive presence in the marketplace", Lyons said that the complaints of some commercial rivals should not always be allowed to dominate.
"When a commercial competitor of the colossal scale, ambition and financial muscle of Sky attempts to position itself as seriously threatened by the very existence of the BBC, it can be hard to take such charges entirely seriously," he said.